MONTANA
STATE

This "cover" page added by the Internet Archive for formatting purposes

Digitized by the Internet Archive

in 2008 with funding from

Montana State Library

http://archive.org/details/beaverheadrivercOOnelsrich

s

639.313

F2BRC

1977

MONTANA STATE LIBRARY

STATE DOCUMENTS COLLECTION

MAY 1 6 1978

MONTANA ««

930 E Lyndale Ave.
Helena, Montana 59601
BEAVERHEAD RIVER

AND

CLARK CANYON RESERVOIR FISHERY STUDY

by

Frederick A. Nelson
Montana Department of Fish and Game
Bozeman, Montana 59715

M

Prepared for

U.S. Department of the Interior

Bureau of Reclamation

Washington, D. C. 20240

Contract No. 14-06-600-8790

1

MSL Ji9 **

1 3 1985
SEP 2 4 v

OCT 1 2 2006

1

ACKNOWLEDGEMENTS

This report represents the culmination of a study utilizing data
collected by fisheries biologists of the Montana Department of Fish and
Game between 1966 and 1976. Much of the data was collected and sum-
marized by Dr. William Miller, Norman W. Peterson, Barry M. Schaplow,
and Harold R. Stevenson. Other department biologists who participated
in the study are Allen A. Elser, Ronald G. Marcoux, Bruce May, E.
Richard Vincent and Alfred H. Wipperman. The manuscript was reviewed
by Eley P. Denson, Dr. Calvin M. Kaya, Richard H. Kennedy, Ronald G.
Marcoux, Dr. William Miller, Norman W. Peterson, and Donald L. Tennant.
Dr. Donald R. Reichmuth, Marietta Smith, and Dr. Kenneth J. Tiahrt
assisted with the analyses of data.

The Bureau of Reclamation provided the Montana Department of
Fish and Game $138,000 to conduct this study. Additional funding
was provided through Dingell -Johnson Project F-9-R, Montana.

I

PART I

FISHERY AND FLOW RELATIONSHIPS IN THE BEAVERHEAD RIVER
BELOW CLARK CANYON RESERVOIR

(

TABLE OF CONTENTS

Page

PART I

TABLE OF CONTENTS 3

LIST OF TABLES 4

LIST OF FIGURES 8

ABSTRACT 10

CONCLUSIONS 12

RECOMMENDATIONS 15

INTRODUCTION 17

METHODS 17

DESCRIPTION OF STUDY AREA 18

RESULTS 28

Trout-Flow Relationships in the Hildreth Section 28

Numbers of trout (Hildreth Section) 29

Brown Trout . , 29

Rainbow Trout 34>

Biomass of Trout (Hildreth Section) 44

Brown Trout , 44

Rainbow Trout 47

Annual Rates of Population Changes (Hildreth Section). 49

Growth of Trout (Hildreth Section) 53

Trout-Flow Relationships in All Sections 53

Comparison of trout populations in 1975 and 1976 ... 55

Comparison of trout populations between sections ... 59

Comparison of trout populations throughout the study . 64

Physical and Hydraulic Characteristics 65

DISCUSSION 69

LITERATURE CITED 76

APPENDIX 77

LIST OF TABLES

TABLE 1 .

TABLE 2.

TABLE 3.

TABLE 4.

TABLE 5.

TABLE 6.

TABLE 7.

TABLE 8.

TABLE 9.

Selected physical and cover characteristics of the
study sections in the Beaverhead River

Mean flows (cfs) between successive spring and fall
estimates of trout populations in the Hildreth Section
of the Beaverhead River between 1966 and 1976. Range
of average daily flows in parenthesis

Estimated numbers of brown trout by age groups in the
Hildreth Section (6455 ft) of the Beaverhead River
between 1966 and 1976. 80% confidence interval in
parenthesis

Estimated numbers of brown trout in the 1965 through
1974 year classes in the Hildreth Section (6455 ft)
of the Beaverhead River

Correlation coefficients for the multiple linear
relationships between the spring estimates of numbers
of age IV and older brown trout and mean irrigation and
non irrigation flows in the Hildreth Section of the
Beaverhead River

Estimated numbers of rainbow trout by age groups in the
Hildreth Section (6455 ft) of the Beaverhead River
between 1966 and 1976. 80% confidence interval in
parenthesis

Estimated numbers of rainbow trout in the 1965 through
1974 year classes in the Hildreth Section (6455 ft) of
the Beaverhead River

Correlation coefficients for the multiple linear
relationships between fall estimates of numbers of age
III and age IV and older rainbow trout and mean
irrigation and nonirrigation flows in the Hildreth
Section of the Beaverhead River

Correlation coefficients for the multiple linear
relationships between the spring estimates of numbers
of age II and older rainbow trout and mean fall to
spring flows in the Hildreth Section of the Beaverhead
River

Page
25

30

32

34

38

41 1

80

44

LIST OF TABLES

Page

TABLE 10. Relationship between relative change in the fall to

spring estimates of number of age I and older rainbow

trout and mean fall to spring flows (cfs) in the

Hildreth Section of the Beaverhead River. Range of

average daily flows in parenthesis 45

TABLE 11. Estimated biomass (lbs/6455 ft) of brown trout by age
groups in the Hildreth Section of the Beaverhead River
between 1966 and 1976. 80% confidence interval in
parenthesis 46

TABLE 12. Estimated biomass (lbs/6455 ft) of brown trout in the
1965 through 1974 year classes in the Hildreth Section
of the Beaverhead River 47

TABLE 13. Estimated biomass (lbs/6455 ft) of rainbow trout by age
groups in the Hildreth Section of the Beaverhead River
between 1966 and 1976. 80% confidence interval in
parenthesis 48

TABLE 14. Estimated biomass (lbs/6455) of rainbow trout in the 1965
through 1974 year classes in the Hildreth Section of
the Beaverhead River 49

TABLE 15. Annual rates of population change (%) of brown trout

by age groups in the Hildreth Section of the Beaverhead

River between 1967 and 1976 49

TABLE 16. Annual rates of population change (%) of rainbow trout
by age groups in the Hildreth Section of the Beaverhead
River between 1966 and 1975 50

TABLE 17. Relationship between tag returns of brown and rainbow
trout by anglers and mean April through September flows
(cfs) for the Hildreth Section of the Beaverhead River.
Number of tag returns in parenthesis 52

TABLE 18. Mean weights (lbs) by age groups of brown and rainbow

trout captured by electrofishing in the Hildreth Section

of the Beaverhead River during the 1966 through 1976

sampling periods 52

TABLE 19. Estimated numbers (N/1000 ft) and biomass (lbs/1000

ft) of brown trout by age groups in nine sections of the

Beaverhead River during Spring, 1975 and 1976. 80%

confidence interval in parenthesis 81

LIST OF TABLES

TABLE 20. Estimated numbers (N/1000 ft) and biomass

(lbs/1000 ft) of rainbow trout by age groups in
nine sections of the Beaverhead River during Spring,
1975 and 1976. 80% confidence interval in
parenthesis

TABLE 21. Minimum and maximum average daily flows (cfs) between
April land March 31 in 9 sections of the Beaverhead
River in 1973-74, 1974-75, and 1975-76

TABLE 22. Relationship between annual rates of population

decrease (%) of age II and older brown trout and mean
flows (cfs) in a series of study sections in the
Beaverhead River between spring, 1975 and spring
1976. Range of average daily flows in parenthesis. .

TABLE 23. Summary of turbidity (JTU) measurements for 10
sites on the Beaverhead River. Samples were
collected between March 13, 1975 and February 27,
1976

Page

82

55

57

62

TABLE 24. Summed results for kick samples collected in a series
of study sections in the Beaverhead River on October
22, 1975. Three samples were collected in a riffle
in each section 64

TABLE 25. Estimated numbers (N/1000 ft) and biomass (lbs/1000 ft)
of brown trout by age groups in the Pipeorgan Section
(8513 ft) of the Beaverhead River between 1967 and
1976. Rainbow trout were too scarce to estimate.
80% confidence interval in parenthesis 83

TABLE 26. Estimated numbers (N/1000 ft) and biomass (lbs/1000 ft)
of brown trout by age groups in the Barretts Upstream
Section (5792 ft) of the Beaverhead River between 1967
and 1976. Rainbow trout were too scarce to estimate.
80% confidence interval in parenthesis 84

TABLE 27. Estimated numbers (N/1000 ft) and biomass (lbs/1000 ft)
of brown trout by age groups in the Wheat Section
(9908 ft) of the Beaverhead River between 1968 and
1976. 80% confidence interval in parenthesis .... 84

LIST OF TABLES

TABLE 28. Estimated numbers (N/IOOO ft) and biomass (lbs/1000

ft) of rainbow trout by age groups in the Wheat Section
(9908 ft) of the Beaverhead River between 1968 and
1976. 80% confidence interval in parenthesis ....

TABLE 29. Estimated numbers (N/1000 ft) and biomass (lbs/1000 ft)
of brown trout by age groups in the Blaine Section
(17837 ft) of the Beaverhead River between 1971 and
1976. Rainbow trout were too scarce to estimate. 80%
confidence interval in parenthesis

TABLE 30. Comparison of the spring estimates of numbers (N/1000
ft) of age II and older brown trout in the Pipeorgan
(8513 ft) and Hildreth (6455 ft) Sections of the
Beaverhead River between 1968 and 1976. 80% confidence
interval in parenthesis

TABLE 31. Comparison of the fall estimates of numbers (N/1000 ft)
of Age I brown trout in the Pipeorgan (8513 ft) and
Hildreth (6455 ft) Sections of the Beaverhead River
between 1967 and 1973

Page

85

85

66

66

TABLE 32. Mean values of hydraulic characteristics at various
flows (cfs) in percent of the mean values at 1000 cfs
for pools, runs, and riffles in the Hildreth Section
of the Beaverhead River 86

TABLE 33. Approximate flows (cfs) at which various hydraulic

characteristics are reduced by 50% in relation to the

mean values at 1000 cfs in pools, runs, and riffles

in a series of study sections in the Beaverhead

River 87

TABLE 34. Numbers of brown and rainbow trout > 5.0 lbs captured
by electrofishing in the Hildreth Section (6455 ft) of
the Beaverhead River between 1966 and 1976

TABLE 35. Distribution of the average daily flows between

successive fall and spring estimates of trout populations
in the Hildreth Section of the Beaverhead River . . .

70

LIST OF FIGURES

FIGURE 1. Average monthly discharges (cfs) + 1 standard

deviation in the Beaverhead River at the U.S.G.S.
gage at Barretts before impoundment (1950-63 water
years) and after impoundment (1965-75 water years) .

FIGURE 2. Average monthly discharges (cfs) + 1 standard

deviation in the Beaverhead River at the U.S.G.S.
gage near Twin Bridges before impoundment (1951-63
water years) and after impoundment (1965-75 water
years)

FIGURE 3. Mean daily water temperatures at three stations on
the Beaverhead and Red Rock Rivers, 1971-72

FIGURE 4. Map of the Beaverhead River showing the approximate
location of the 9 study sections

FIGURE 5. Spring estimates of numbers (N/6566 ft) and biomass
(lbs/6455 ft) with 80% confidence intervals of age II
and older brown trout in the Hildreth Section of the
Beaverhead River between 1967 and 1976

FIGURE 6. Relationships between the fall estimates of numbers
(N/6455 ft) of age I brown trout and the estimated
numbers of age II and age III brown trout of the same
year class in succeeding springs in the Hildreth
Section of the Beaverhead River

Page
21

22
23
27

33
35

FIGURE 7. Average daily flows (cfs) during the 1964 through 1973
spawning periods (September 15 - October 31) of
brown trout in the Hildreth Section of the Beaverhead
River. Estimated number of age I brown trout (N/6455
ft) two years later is under each plot 37

FIGURE 8. Relationship between the log variance of the average
daily flows (cfs) during the brown trout spawning
period (September 15 - October 31) and the estimated
number of age I brown trout two years later in the
Hildreth Section of the Beaverhead River 79

FIGURE 9. Fall estimates of numbers (N/6455 ft) and biomass

(lbs/6455 ft) with 80% confidence intervals of age I

and older rainbow trout in the Hildreth Section of

the Beaverhead River between 1966 and 1975 39

LIST OF FIGURES

Page

FIGURE 10. Relationship between the fall estimates of numbers
(N/6455 ft) of age I rainbow trout and the estimated
numbers of age II rainbow trout of the same year class
in the following fall in the Hildreth Section of the
Beaverhead River 41

FIGURE 11. Average daily flows (cfs) during the 1965 through 1974
spawning periods (March 1 - April 30) of rainbow
trout in the Hildreth Section of the Beaverhead River.
Estimated number of age I rainbow trout (N/6455 ft)
18 months later is under each plot 43

FIGURE 12. Relationship between the annual rates of population
decrease (%) of age III and older rainbow trout and
mean nonirrigation flows (cfs) in the Hildreth Section
of the Beaverhead River 51

FIGURE 13. Relationship between the estimated numbers (N/6455 ft)
and the mean weights (lbs) of age II brown trout during
the spring in the Hildreth Section of the Beaverhead
River 54

FIGURE 14. Mean weights (lbs) of age II brown trout in a series
of study sections in the Beaverhead River in spring,
1975 and spring, 1976 58

FIGURE 15. Estimated numbers (N/1000 ft) of age II and older

brown, rainbow, and total trout in a series of study
sections in the Beaverhead River in spring, 1975 and
spring, 1976 60

FIGURE 16. Estimated biomass (lbs/1000 ft) of age II and older
brown, rainbow, and total trout in a series of study
sections in the Beaverhead River in spring, 1975 and
spring, 1976 61

FIGURE 17. Mean values of physical and hydraulic characteristics
at various flows (cfs) in percent of the mean values
at 1000 cfs for 20 cross sections in the Hildreth
Section of the Beaverhead River 68

FIGURE 18. Relationship between the annual rates of population
decrease (%) of age III and older rainbow trout and
the number of average daily flows <150 cfs between
successive fall population estimates 72

10

ABSTRACT

The effects of varied flow patterns on trout populations and the
physical and hydraulic characteristics in a 6455 ft section of the
upper Beaverhead River below Clark Canyon Reservoir, Montana, were
measured between 1966 and 1976. Population estimates were made using
electrofishing techniques. The WSP (Water Surface Profile) computer
program of the Bureau of Reclamation was used to predict physical and
hydraulic values at various flows.

The survival of age III and older rainbow trout was directly related
to the magnitude of flows during the nonirrigation season (approximately
October 15 to April 15). During this period, water for irrigation is
being stored in the reservoir and releases into the river are minimal.
Results of this study suggest that average daily flows greater than
approximately 250 cfs are needed to provide a high quality, trophy
rainbow trout fishery. Flows during this study were not sufficiently
reduced to adversely affect the survival of older brown trout.

Poor reproductive success was the major factor limiting the total
numbers and biomass of trout throughout much of this study. Repro-
ductive success, as measured by the estimated numbers of age I trout,
appears to be related to flow patterns during the brown and rainbow
trout spawning periods. With flow releases favorable to both repro-
duction and the survival of older trout, the upper Beaverhead River
is capable of supporting greater numbers and biomass of trout of all
age groups than those which existed throughout much of this study.

The physical and hydraulic characteristics most affected by flow
reductions were cross sectional area and current velocity. Top
width and wetted perimeter were least affected. The rate of loss for

11

all six of the measured characteristics was greatly accelerated at flows
less than approximately 200 cfs.

An evaluation of biological, chemical, discharge, and habitat
data for a series of nine study sections suggests that other factors
are operating in conjunction with flows to limit numbers and biomass
of trout in much of the Beaverhead River below the confluence of
Grasshopper Creek.

12
CONCLUSIONS
The following conclusions pertain to the Beaverhead River between
Clark Canyon Dam and the confluence of Grasshopper Creek (mile 0 to 12).

1. The survival of age III and older rainbow trout is directly
related to the magnitude of flows during the nonirrigation
season (approximately October 15 to April 15). During this
period, Clark Canyon Reservoir is storing water for irrigation
and releases into the Beaverhead River are minimal. Results
of this study suggest that one average daily flow <^ 150 cfs

is sufficient to adversely affect the survival of older
rainbow trout. Average daily flows greater than approximately
250 cfs are needed to provide a high quality, trophy rainbow
trout fishery.

2. Flows were not sufficiently reduced during this study to
adversely affect the survival and numbers of older brown
trout.

3. The reproductive success of brown trout, as measured by the
estimated numbers of age I brown trout, appears to be
related to flow patterns during the brown trout spawning
period. Spawning flows devoid of violent fluctuations and
gradually decreasing to a minimum of approximately 150 cfs
during the 47 day spawning period (September 15 through
October 31) yielded the highest estimates of numbers of
age I brown trout two years later. The number of age I
brown trout in turn was the major factor influencing numbers
of age II and age III brown trout in succeeding years.

13

%. Spawning flows also appear to affect the numbers of age I

rainbow trout. Spawning flows devoid of violent fluctuations
and increasing from approximately 200 cfs to approximately
700 cfs during the 61 day spawning period (March 1 through
April 30) yielded the highest estimates of age I rainbow
trout 18-months later. The number of age I rainbow trout
in turn was the major factor influencing the number of
age II rainbow trout the following fall.

5. Poor reproductive success was the major factor limiting
the total numbers and biomass of trout throughout much of
this study.

6. With flow releases favorable to both reproduction and the
survival of older trout, this portion of the Beaverhead River
is capable of supporting greater numbers and biomass of
trout of all age groups than those which existed throughout
much of this study.

7. Relationships between the magnitude of flows and trout growth
were not demonstrated during this study.

8. The physical and hydraulic characteristics most affected by
flow reductions were cross sectional area and current
velocity. Top width and wetted perimeter were least
affected. The rate of loss for all six of the measured
characteristics was greatly accelerated at flows less than
approximately 200 cfs.

14

The following conclusions pertain to the Beaverhead River below
the confluence of Grasshopper Creek (mile 12 to 69).

1. Average daily flows less than 25 cfs, which have occurred
in sections of the river 18 and 38 miles below Clark Canyon
Dam, have undoubtedly affected aquatic life.

2. The numbers and biomass of trout in only one of six study
sections in this portion of the Beaverhead River increased
following the flow increases that occurred between Spring,
1975 and 1976.

3. Other factors, notably sediment, cover, and toxic metals,
are operating in conjunction with flow to limit trout
populations in much of the river below Grasshopper Creek.

4. Sedimentation appears to be a major factor limiting trout
populations in the lower reaches of the river (mile 52 to
69).

15
RECOMMENDATIONS

1. The management of the rainbow trout provides the greatest potential
for enhancing the upper Beaverhead fishery. Emphasis should be
directed towards this species.

2. Violent fluctuations in the flow releases at Clark Canyon Dam
should be avoided during the rainbow and brown trout spawning
periods. Results of this study suggest that flow releases
decreasing and then increasing by 125 cfs or more within a two
week period are undesirable during spawning. Further evaluation
of the relationship between the reproductive success of brown

and rainbow trout and the flow patterns during spawning is needed.

3. A minimum flow release of approximately 200 cfs is needed at
Clark Canyon Dam to provide a high quality, trophy rainbow
trout fishery in the upper Beaverhead River.

4. Average daily flows of 50 cfs throughout the Beaverhead River
are adequate only for the short term survival of trout. A
reduction in trout numbers can be expected if average daily
flows of approximately 50 cfs are maintained for periods greater
than a few days.

5. The Montana Department of Fish and Game should continue to
monitor trout populations semiannually in the upper Beaverhead
River. If reproduction problems in the upper river are eliminated
in future years, flow patterns may have a more pronounced influence
on trout populations.

6. A flow management plan to minimize the impact of flow reductions
and fluctuations on trout populations should be developed for
all portions of the Beaverhead River.

16

7. The sediment-flow-biological relationships should be evaluated for
the Beaverhead River. This would provide background data for the
evaluation of other projects which divert a large volume of flow
during the spring runoff period.

8. Further evaluation of the effects of toxic metals, cover, and

food supply on trout populations in the Beaverhead River is needed.

'

*

17

INTRODUCTION

The effects of flow releases from Clark Canyon Reservoir on trout
populations in the Beaverhead River have been investigated by the Montana
Department of Fish and Game since 1966. The purpose of this study, which
was funded by the Bureau of Reclamation, was to measure changes in the
fishery caused by varied flow patterns, particularly during the non-
irrigation season when irrigation water is stored in the reservoir. The
study will provide biological criteria for recommending flows that will
maintain a desirable river fishery.

METHODS

Trout populations in the Beaverhead River were sampled using a boat
mounted electrofishing unit. The weight and total length of each
captured fish were measured to the nearest 0.02 pound and 0.1 inch,
respectively. Scales were taken for age determination. Some young-of-
the-year and yearling trout were permanently marked by removing an
adipose or pelvic fin. Trout >8 inches were tagged with numbered T-tags.
All trout were marked with a partial fin clip and released for mark-
recapture population estimates. As many as three marking and two
recapture runs were made per estimate. Estimates of numbers, biomass,
age structures, mean weights by age groups, and appropriate confidence
intervals were calculated using methods summarized by Vincent (1971)
and adapted for computer analyses.

Average daily flows at each study section were derived from U.S.G.S.
records and known daily irrigation withdrawals. A plot of the average
daily flows for each study section was obtained using computer techniques
For statistical purposes, the average daily flows between successive fall

and successive spring population estimates were averaged to obtain a
mean flow. These periods were further divided into irrigation and
nonirrigation seasons and a mean flow obtained for each. Between
successive spring estimates, April 15 through October 15 was termed
the irrigation season. The remaining period was the nonirrigation
season. Between fall estimates, April 15 to the time of the fall
estimate was the irrigation season. The nonirrigation season occurred
from the time of the preceding fall estimate through April 14.

The WSP (Water Surface Profile) computer program of the Bureau of
Reclamation was used to predict hydraulic values at various flows.
The procedures followed are outlined by Spence (1975) and Dooley (1976).

The percentage of willow cover along the banks of the Beaverhead
River was assessed using color infra-red photography and a stero-
comparator. Photogrammetry work was done by graduate students at
Colorado State University. Sinuosity and section length were determined
from aerial photographs using computer facilities of the Civil
Engineering Department, Montana State University.

Simple and multiple linear regressions were derived on a computer
using the method of least squares. Unless noted, the term significant
refers to statistical significance at the 5% level (p <_ .05).
DESCRIPTION OF STUDY AREA

Prior to the construction of Clark Canyon Reservoir in 1964,
the Beaverhead River originated at the confluence of the Red Rock
River and Horse Prairie Creek. The Beaverhead River now originates at
the outlet of Clark Canyon Reservoir and flows for 69 miles to its
confluence with the Big Hole River near Twin Bridges, Montana. Between

19

Clark Canyon Dam and the Big Hole River, the Beaverhead drops 850
feet, an average fall of 11.8 ft/mile. River elevations at the dam
outlet and mouth are 5450 and 4600 ft, respectively.

Chemical analyses of water samples collected at sites 0.25, 6,
15, and 27 miles below Clark Canyon Dam in the summer of 1972 yielded

these mean results (Smith, 1973):

Site (miles)

"0725 6~ 15 ~2T

Turbidity (JTU)
Conductivity (umhos @ 25C)
pH

Dissolved Oyxgen (ppm)
Total Alkalinity (ppm CaC03)
Total Hardness (ppm CaC03)
Ammonia (ppm NH3-N)
Nitrate (ppm N0~-N)

Nitrite (ppm N0~-N)

-3
Orthophosphate (ppm PO^ )

4

4

7

5

565

572

555

617

8.1

8.2

8.2

8.1

9.6

9.7

9.3

10.0

198

199

190

218

220

230

216

252

.14

.08

.05

.02

.057

.110

.089

.285

.015

.018

.015

.006

.11

.10

.08

.05

Smith (1973) evaluated the effects of Clark Canyon Reservoir on
some chemical characteristics of the Beaverhead River. The reservoir
had its greatest effect upon the dissolved oxygen levels in the tail-
waters. DO levels in the tailwaters were well above saturation levels
while the reservoir tributaries had mean saturation levels below or
near saturation. He concluded that critically low DO due to bottom
releases from the reservoir is not likely to become a problem. Compared
to the reservoir tributaries, ammonia, nitrite and pH in the tailwaters
increased sufficiently to be considered changed by the reservoir. The
increase in ammonia and nitrite was attributed to the reduction of

20

nitrates and/or the decomposition of organic matter in the deeper parts
of the reservoir.

Pre- and postimpoundment flows at the Barretts gage, located
approximately 15 miles below the dam, are compared in Figure 1. From
October through March, Clark Canyon Reservoir stores water, therefore,
inflows exceed outflows. However, average monthly discharges (October
through March) during preimpoundment years were not greater than those
after impoundment. The similarity of average monthly discharges (October
through March) is a result of above normal inflows in postimpoundment
years. Although pre- and postimpoundment average monthly discharges
were similar (October through March), the variability of average monthly
discharges was greatest after impoundment. The major impact of the
reservoir on the upper reaches of the river was to extend the high flow
period an additional five months from April through September. This
extension occurs at the expense of October through March flows.

Pre- and postimpoundment flows at the near Twin Bridges gage,
located approximately 52 miles below the dam, are compared in Figure 2.
The major effect of the reservoir on the lower reaches of the river was
to increase flows during the preimpoundment low flow months of May,
July, August and September. November and late spring remained high

flow periods.

The influence of Clark Canyon Reservoir on water temperatures in
the Beaverhead River is shown in Figure 3. The Grant and Blaine
stations are located on the Beaverhead River approximately 0.25 and
52 miles, respectively, below the dam. The Red Rock River, which is
the major tributary to the reservoir, reflects the temperature of the

21

1000-|

900'
- 800

o

<

5 700'

5

5 600

z

z

O 500

s

i 400

>

< 300

200'

100

PREIMPOUNDMENT

M^V

FIGURE 1. Average monthly discharges (cfs) + 1 standard deviation in

the Beaverhead River at the U.S.G.S. gage at Barretts before
impoundment (1950-63 water years) and after impoundment
(1965-75 water years).

22

O 800

PREIMPOUNDMENT

\A\^

•

1000

900

„, 800i

o

<

5 700,

5

v 600'

z

I 500'

< 400'

>
<

300^

200-

100

POSTIMPOUNDMENT

/

\

\

\

FIGURE 2. Average monthly discharges (cfs) + 1 standard deviation in
the Beaverhead River at the U.S.G.S. gage near Twin Bridges
before impoundment (1951-63 water years) and after impound-
ment (1965-75 water years).

?

23

(;

<■£■■

I

/

'-K

. -\ ;

o m a

\

1

-

<

5

o

-I

°

o

. o

1 m
J

j

s- en

d) i—

2 -^

(J

>> o

kd) 3dniVU3dlM31

24

Beaverhead River before impoundment. The primary effect of the reservoir
has been to reduce fluctuations in water temperature near the dam's
outfall. Diel fluctuations in the summer of 1971 and 1972 at the Grant
station had a range of approximately 2F while those at the Red Rock River
station had a range of approximately 9F (Smith, 1973). From July through
September, 1972, mean water temperatures at the Grant station exceeded
those at the Red Rock River station by approximately 5F. This is a
result of the solar heating of the reservoir.

Water temperature fluctuations at the Blaine and Red Rock River
stations were similar, even though these two stations are separated by
a distance of over 50 miles. Water temperature fluctuations at these
two stations were similar because they both reflect variations in air
temperatures.

Additional water temperature data was collected on the Beaverhead
River between 1963 and 1968 at three stations, located approximately
0.25, 15, and 52 miles below the dam. The maximum water temperature
recorded at the three stations during this period was 76.5 F. It
would appear that water temperatures in the Beaverhead River are
not high enough to adversely affect the survival of trout. The
temperature data is presently being analyzed using a computer program
developed by the Civil Engineering Department, Montana State University.
The results will be presented in a separate report.

Nine study sections were selected along the Beaverhead River
primarily on the basis of differences in annual flow patterns. Due to
irrigation withdrawals, annual flow patterns, as indicated by Figures
1 and 2, vary greatly between the upper and lower river. Selected
physical and cover characteristics of these sections are given in Table 1.

25

TABLE 1. Selected physical and cover characteristics of the study
sections in the Beaverhead River.

Willow Bank

Section

, , 9 , W II I UW Ddll

Sinuosity Gradient(%F Riprap(%)-' Cover(%)3/

Grant (.01) 1.19 .21 14.6

Hildreth (2) - - 2.3 77

Hildreth Extended (2) 1.32 .33 1.2 67

Pipeorgan (8) - - 3.4 50

Pipeorgan Extended (8) 1.62 .24 5.2 48

Barretts Upstream (15) - - 3.1 37

Barretts Ups. Ext. (15) 1.22 .31 15.4 27

Barretts Downstream (18) 1.19 .29 0.7 50

Wheat (27) 1.60 .14 6.1 30

Westside Canal (29) 1.33 .24 10.5 29

Anderson Lane (38) 1.46 .17 0.0 54

Blaine (52) 1.66 .12 4.3 34

1/ % = ^^- X 100

— run

oi v Total length of riprap on both banks x ^Q0

— 2 x section length

,. 0, _ Total length of willow cover on both banks x -j qq

— 2 x section length

26

These sections are indicated on the map (Figure 4) and are described as
follows:

Section 1 . Grant (0.1).- This section began approximately 0.1
mile below Clark Canyon Dam. Section length was 8589 feet.

Section 2. Hildreth (2). This section began approximately 2 miles
below Clark Canyon Dam at the 1-15 bridge. Section length was 6455
feet. In the spring of 1975 and 1976, the lower boundary was extended
an additional 6048 feet downstream.

Section 3. Pipeorgan (8). This section began approximately 8 miles
below Clark Canyon Dam at the 1-15 bridge. Section length was 8513 feet.
In the spring of 1975 and 1976, the lower boundary was extended an ad-
ditional 5649 feet downstream.

Section 4. Barretts Upstream (15). This section began approximately
15 miles below Clark Canyon Dam. Section length was 5792 feet. In the
spring of 1975 and 1976, the upper boundary was extended an additional
9866 feet upstream to approximately 0.2 mile below the mouth of Grass-
hopper Creek.

Section 5. Barretts Downstream (18). This section began approximately
18 miles below Clark Canyon Dam at the Barretts diversion dam. Section
length was 10296 feet.

Section 6. Wheat (27). This section began approximately 27 miles
below Clark Canyon Dam. The lower boundary was approximately .25 mile
upstream of the Westside Canal diversion dam. Section length was 9908
feet.

Section 7. Westside Canal (29). This section began approximately
29 miles below Clark Canyon Dam at the Westside Canal diversion dam.
Section length was 14533 feet.

1/ Miles below dam.

27

FIGURE 4. Map of the Beaverhead River showing the approximate location
of the 9 study sections.

28

Section 8. Anderson Lane (38). This section began approximately
38 miles below Clark Canyon Dam at the Anderson Lane bridge. Section
length was 9930 feet.

Section 9. Blaine (52). This section began approximately 52 miles
below Clark Canyon Dam at a point approximately 0.2 mile upstream of
Point of Rocks. Section length was 17837 feet.

Brown trout (Salmo trutta), rainbow trout (Salmo gairdneri ) ,
mountain whitefish (Prosopium williamsoni ) , burbot (Lota lota), white
sucker (Catostomus commersoni), longnose sucker (Catostomus catostomus),
mottled sculpin (Cottus bairdi), and longnose dace (Rhinichthys
cataractae) were common in the study sections of the Beaverhead River.
Brook trout (Salvelinus fontinalis) and mountain sucker (Catostomus
platyrhynchus) were uncommon. Carp (Cyprinus carpio) were common in
the Blaine (52) Section and uncommon elsewhere.

RESULTS
Trout-Flow Relationships in the Hildreth Section

Results for the Hildreth Study Section will be discussed separately.
More flow and fish population data was collected in this section than
in the other study sections.

Numbers and biomass of brown and rainbow trout in the Hildreth
Section were estimated in the spring and fall between October, 1966 and
March, 1976. Fall estimates during this period were made between
September 20 and October 28. Spring estimates were made between March 1
and April 2. Age I and age II trout were the youngest group estimated
in the fall and spring, respectively.

29

Flows, which are completely regulated by Clark Canyon Reservoir, are
summarized for the Hildreth Section in Table 2. Average daily flows were
derived from U.S.G.S. records for the gage near Grant, located 0.4 mile
below the dam. From October through March and April through September,
35 and 45 cfs, respectively, were added to the average daily flows near
Grant to account for accretion. Due to accretion and leakage at the dam,
the lowest instantaneous flow that can occur in the Hildreth Section is
approximately 50 cfs. During the study, average daily flows ranged from
57 to 1365 cfs. Mean irrigation flows ranged from 320 to 870 cfs and
mean nonirrigation flows ranged from 97 to 467 cfs.

Trout populations in the Hildreth Section early in this study may
reflect water quality problems in addition to flows. In January of 1965,
a fish kill attributed to toxic concentrations of hydrogen sulfide in
reservoir releases occurred in the river immediately below the dam
(Wipperman, 1965). Results of field bioassays with caged fish and
censusing with electrofishing gear indicated that the kill was restricted
to approximately the upper 1.5 miles of river. This did not include
the Hildreth Section. Elevated concentrations of hydrogen sulfide were
measured in the summer of 1965 and were believed to have eliminated fish
in the upper 1.5 miles of river, although a fish kill was not documented
(Needam and Wipperman, 1967). A kill also attributed to hydrogen sulfide
occurred in September of 1967 in approximately the upper 3 miles of
river (Wipperman and Elser, 1968). This partially included the Hildreth
Section.
Numbers of trout (Hildreth Section)

Brown Trout

The estimated numbers of brown trout by age groups are given in

30

TABLE 2. Mean flows (cfs) between successive spring and fall estimates of trout
populations in the Hildreth Section of the Beaverhead River between
1966 and 1976. Range of average daily flows in parenthesis. 4

Mean Flow Mean Flow (cfs) Mean Flow (cfs)
Fall to Fall No. of Days (cfs) Irrigation Nonirrigation

112
261
377
292
394
467
316
200
97

230
336
347
356
421

1966-67

371

215
(82-692)

325

1967-68

374

372
(84-858)

484

1968-69

363

535
(105-1115)

687

1969-70

369

478
(107-1033)

654

1970-71

358

637
(169-1020)

870

1971-72

342

607
(139-1105)

768

1972-73

378

453
(167-1115)

616

1973-74

379

433
(57-1065)

675

1974-75

Spring to Spring

376

451
(60-1365)

773

1967-68

398

272

(110-692)

320

1968-69

350

416

(84-858)

489

1969-70

358

523
(105-1115)

690

1970-71

365

512
(155-1033)

665

1971-72

370

643
(139-1020)

868

1972-73

-

-

-

1973-74

-

-

-

1974-75

382

394
(57-1065)

681

1975-76

351

624
(101-1365)

792

126
439

'.I

Table 3. Fall estimates of age II and older brown trout are inflated due
to the upstream movement of spawning fish into the Hildreth Section. While
fall estimates of age groups II and older may reflect numbers of brown
trout of spawning age, they do not reflect numbers of resident trout.
Fall estimates of age I brown trout are valid. These fish are sexually
immature and, therefore, considered residents.

Spring estimates of age II and older brown trout are compared in
Figure 5. Total numbers decreased substantially between 1967 and 1968,
remained relatively stable between 1968 and 1974, then increased between
1974 and 1976. An elevated mortality of age III and older trout was
responsible for the decrease between 1967 and 1968. The increase
between 1974 and 1976 reflects strong 1973 and 1974 year classes.

Strengths of the 1965 through 1974 year classes of brown trout are
shown for an 18-month period in Table 4. The data suggests that some
movement of brown trout into the Hildreth Section did occur following
a poor yearling crop. Although movement may have occurred, the number
of yearlings still had a strong influence on year class strength in
succeeding years. Simple linear regression analyses (Figure 6) show
that the number of yearlings explain 83 and 84f, respectively, of the
annual variation in numbers of age II and age III brown trout in succeeding
years. Numbers of age IV and older brown trout were not related to
the numbers of age III and older brown trout the previous spring nor
to the magnitude of mean irrigation and nonirrigation flows (Appendix
Table 5).

32

TABLE 3. Estimated numbers of brown trout by age groups in the Hildreth Section

(6455 ft) of the Beaverhead River between 1966 and 1976. 80% confidence
interval in parenthesis. V

Age

Group

Total

I

II

III

IV & Older

Fall

55

447

739(±154)

1966

LJ/

1967

Spring
Fall

391/

541/

- 545 "-

717V

771 (±135)

584(±155)

1968

Spring
Fall

474

65
213

133
131

173
25

371 (±69)
843(±157)

1969

Spring
Fall

438

237
512

111

151

171

44

519(±147)
1145(±281)

1970

Spring
Fall

164

151
613

205
141

144
56

500(±74)
974(±234)

1971

Spring
Fall

141

237
107

133
339

165
264

535(±100)
851 (±172)

'

1972

Spring
Fall

158

74
86

121
167

209
147

404 (±53)
558(±116)

1973

Spring
Fall

57

165

234

187

643(±123)

1974

Spring
Fall

908

32
201

90
272

195
203

317(±50)

1584 (±286)

1975

Spring
Fall

-

467

61

142

670(±82)

1976

Spring

-

624

420

139

1183(±285)

1/Age groups separated by length-frequency distribution.

33

i/P-N

FIGURE 5. Spring estimates of numbers (N/6455 ft) and biomass (lbs/6455 ft)
with 80% confidence intervals of age II and older brown trout
in the Hildreth Section of the Beaverhead River between 1967
and 1976.

34

TABLE 4. Estimated numbers of brown trout in the 1965 through 1974 year

classes in the Hildreth Section (6455 ft) of the Beaverhead River.

Fall

Spring

Spring

Year Class

Aq

e Group I

Age Group

II

Age

Group III

1965

55

54

133

1966

39

65

111

1967

474

237

205

1968

438

151

133

1969

164

237

121

1970

141

74

-

1971

158

-

90

1972

57

32

61

1973

908

467

420

1974

-

624

-

The plot of average daily flows for the Hildreth Section for the
1964-76 period was visually inspected to determine if the extreme variation
in the estimated numbers of age I brown trout was related to the magnitude
of flows during the 17-month period between fry emergence and the fall
yearling estimate. Visual inspection showed the highest yearling estimate
(908/6455 ft) followed a low winter flow period. Based on the large
number of age II trout in the spring of 1976, a conservation approximation
of the number of yearling brown trout the previous fall would be 624/6455
ft. This high number of yearlings followed the lowest winter flow
period of the study. Flows during the first summer of growth of the
1967 year class were the lowest during the study, yet year class
strength (474/6455 ft) was relatively high in the fall of 1968. It
appears unlikely that reduced flows during the 17-month period pro-
ceeding the fall estimates were the principle factor depressing numbers
of age I brown trout.

Flows during the brown trout spawning period (September 15 -
October 31) were examined for a possible flow-yearling relationship.
Visual inspection of the plot of average daily flows during the 1964

35

400 600

NUMBERS OF AGE I

BERS OF AGE

FIGURE 6. Relationships between the fall estimates of numbers (N/6455 ft)
of age I brown trout and the estimated numbers of age II and
age III brown trout of the same year class in succeeding springs
in the Hildreth Section of the Beaverhead River.

a/ Indicates significance at 5% level.

36

through 1973 spawning periods (Figure 7) indicates that gradually decreas-
ing flows devoid of violent fluctuations yielded the highest yearling
estimates two years later.

The variance of the 47 average daily flows during each of the 1964
through 1973 spawning periods was calculated. Variance, a coarse measure
of the fluctuation of spawning flows, was regressed against the estimated
number of age I brown trout two years later. The estimated numbers of
age I brown trout in 1966, 1967, and 1968 may reflect water quality
problems in addition to spawning flows. When these three observations
are removed, the variance of spawning flows explains 71% of the annual
variation in numbers of age I brown trout (Appendix Figure 8).

Results of this study suggest the degree of fluctuation of spawning
flows is the dominant factor influencing numbers of age I brown trout,
which in turn influence numbers of age II and age III brown trout in
succeeding years. Numbers of age IV and older brown trout were not
related to the numbers of age III and older brown trout the previous
spring nor to the magnitude of mean irrigiation and nonirrigation flows.
Those factors influencing the numbers of older brown trout, which remained
relatively stable throughout the study, were not identified.

Rainbow Trout

Estimated numbers of rainbow trout by age groups are given in
Table 6. Due to the movement of spawning fish into the Hildreth
Section, spring estimates of age II and older rainbow trout are not
valid estimates of resident fish.

Fall estimates of age I and older rainbow trout are compared in
Figure 9. Total numbers remained relatively stable between 1966 and

:7

<

J

— -

<u

~

Q M-

-

-"

C

^

-

-"

3 o

Ol 3

C CD

- - —

*-> CJ3 C

+J SZ 1/1

U (O •<-

ZJ 4-

r- -C O

it- to S-

so woii Aiira 30va3A»

38

TABLE 6. Estimated numbers of rainbow trout by age groups in the Hildreth Section
(6455 ft) of the Beaverhead River between 1966 and 1976. 80% confidence
interval in parenthesis.

Age Group

I

II

III

IV & Older

Total

1966

Fall

73

39

112(±28)

1967

Spring
Fall

171/

95 1/
qqV

95(±42)
116(±40)

1968

Spring
Fall

81

16
14

53
19

43
16

112(±45)
130(±32)

1969

Spring
Fall

81

99
61

9
27

74
18

182(±51)
187(±41)

1970

Spring
Fall

56

35
41

105
43

61
10

201 (±36)
150(±38)

1971

Spring
Fall

10

40
25

89
69

123
34

252(±59)
138(±51)

1972

Spring
Fall

140

0

30
14

185
60

215(±59)
214(±68)

1973

Spring
Fall

136

114

28

53

331 (±75)

1974

Spring
Fall

997

107
143

107
55

101
15

315(±70)
1210(±253)

1975

Spring
Fall

796

595
281

121
26

96
4

812(±81)
1107(±321)

1976

Spring

-

915

476

161

1552(±328)

1/Age groups separated by length-frequency distribution.

39

\

i/1

♦— r

+-*•

2500"

2000'

i

>

\

1500-

/

tooo-

/

i

500-

^V~\A"

4/

I

66 67 68 69 70 71 72 73 74

FIGURE 9. Fall estimates of numbers (N/6455 ft) and biomass (lbs/6455 ft)
with 80% confidence intervals of age I and older rainbow trout
in the Hildreth Section of the Beaverhead River between 1966
and 1975.

40

1972, increased slightly in 1973, increased substantially in 1974, then ^*

decreased slightly in 1975. Strong 1973 and 1974 year classes were

responsible for the elevated numbers in 1974 and 1975.

Strengths of the 1965 through 1974 year classes are shown for a two

year period in Table 7. The data suggests that some movement of rainbow

trout into the Hildreth Section occurred following a poor yearling crop.

Although movement may have occurred, the number of yearlings still had

a strong influence on the number of age II rainbow trout in the following

fall (Figure 10). Numbers of yearlings explain 83% of the annual

variation in numbers of age II rainbow trout one year later. Numbers of

yearlings had little influence on numbers of age III rainbow trout two

years later. Numbers of yearlings explain only 14% of the annual

variation in numbers of age III rainbow trout. ^

TABLE 7. Estimated numbers of rainbow trout in the 1965 through 1974
year classes in the Hildreth Section (6455 ft) of the
Beaverhead River.

Fall

Fall

Fall

Year Class

Ag

e Group I

Age Group II

Age Group III

1965

73

.

19

1966

17

14

27

1967

81

61

43

1968

81

41

69

1969

56

25

14

1970

10

0

28

1971

140

114

55

1972

136

143

26

1973

997

281

-

1974

796

-

"

Multiple linear regressions were computed to evaluate the relationship
between mean irrigation and nonirrigation flows and numbers of age III
and age IV and older rainbow trout (Appendix Table 8). In combination,
the numbers of age III and older rainbow trout the previous fall and mean

41

2 15o-

400

600
NUMBERS OF AGE I

800

FIGURE 10. Relationship between the fall estimates of numbers (N/6455 ft) of
age I rainbow trout and the estimated numbers of age II rainbow
trout of the same year class in the following fall in the Hildreth
Section of the Beaverhead River.

a/ Indicates significance at 5% level.

42

nonirrigation flows explain 81% of the annual variation in the fall
estimates of age IV and older rainbow trout. The other relationships
were not significant.

Visual inspection of the plot of average daily flows for the 1965-75
period showed that low winter flows were not the principle factor
depressing numbers of yearling rainbow trout during the 16-month period
between fry emergence and the fall estimate. The 1975 estimate of year-
lings (796/6455 ft) followed the lowest winter flow period of the study.
The highest estimate (997/6455 ft) also followed low winter flows.

Average daily flows during the 1965 through 1974 rainbow trout
spawning periods (March 1 - April 30) are compared in Figure 11. Visual
inspection shows that spawning flows devoid of violent fluctuations and
increasing from approximately 200 cfs to approximately 700 cfs during
the 61 day spawning period yielded the highest yearling estimates
18-months later. The estimated number of yearlings in 1966, 1967, and
1968 may reflect water quality problems in addition to spawning flows.

Multiple linear regression analyses (Table 9) suggest the magnitude
of winter flows partially determines the numbers of spawning age rainbow
trout present in the spring. In combination, numbers of age I and older
trout in the fall and the mean flows between fall and spring estimates
explain 94% of the annual variation in spring estimates of numbers of
age II and older rainbow trout. When mean fall-spring flows were less
than approximately 275 cfs, estimates of numbers decreased between fall
and spring and increased when mean flows were greater than approximately
275 cfs (Table 10). The lowest mean winter flow (94 cfs) produced the
greatest decrease (33%).

43

~Y

en CD

OJ

C I/O

+->

IB

s_ jr

i —

r: 4->

-o CD

to

S-

c

— -.-o

+->

1/1 1 —

c

4- -i-

o

i— ri LT)

•r- o ^a-

« S-U3
X) +-> ~^

CD o
n3 .Q +->

(StO) MOU A1IVQ 30V83A?

44

TABLE 9. Correlation coefficients for the multiple linear relationships

between the spring estimates of numbers of age II and older

rainbow trout and mean fall to spring flows in the Hildreth
Section of the Beaverhead River.

Partial Correlation Coefficients
No. of I & Older

Rainbow Trout . Mean Flow Multiple

Dependent Variable Previous Fall Fall-Spring Correlation {r]

No. of II & Older -., , , ,,

Rainbow Trout - Spring .9658-' .85101' .971 2— 7

]_/ Indicates significance at 5% level.

Limited evidence suggested spawning flows influenced numbers of
yearling rainbow trout. The number of yearlings in turn determined the
number of age II rainbow trout one year later. Numbers of age IV and
older rainbow trout were influenced by the magnitude of mean flows during
the nonirrigation season.
Biomass of Trout (Hildreth Section)

Brown Trout

Spring and fall biomass estimates of brown trout by age groups are
given in Table 11. Spring biomass estimates of age II and older brown
trout are compared in Figure 5. Biomass decreased between 1967 and 1968,
remained relatively stable between 1968 and 1975, then increased in 1976.
The decrease between 1967 and 1968 reflects an elevated mortality of
age III and older brown trout. Strong 1973 and 1974 year classes
primarily account for the increase in 1976.

The biomass of the 1965 through 1973 year classes are shown for an
18-month period in Table 12. Significant relationships between the
magnitude of mean flows during the irrigation and nonirrigation seasons

45

TABLE 10. Relationship between relative change in the fall to spring estimates of
number of age I and older rainbow trout and mean fall to spring flows
(cfs) in the Hildreth Section of the Beaverhead River. Range of average
daily flows in parenthesis.

Number Mean Flow Relative change in no. of rainbow trout (%) 1/
Fall-Spring of Days Fall-Spring Fall-Spring

1974-75

159

1966-67

148

1973-74

156

1967-68

175

1969-70

146

1968-69

151

1970-71

142

1971-72

153

1975-76

134

94
60-123)

111
82-127)

184
127-260)

257
125-365)

305
107-403)

340
160-523)

389
175-460)

423
139-660)

480
335-587)

-33
-15

- 5

- 3
7

40
68

68?/

40

1/ * _ No. age II & older (spring)

No,

No. age I & older (fall)

age I & older (fall) ,

,11 \ x

00

2/ = % change of age II & older rainbow trout. Valid spring estimate of

age II trout not available.

46

TABLE 11. Estimated biomass (lbs/6455 ft) of brown trout by age groups
in the Hildreth Section of the Beaverhead River between 1966
and 1976. 80% confidence interval in parenthesis.

Age Group

Total

I

II

III

IV & Other

1966

Fall

Spring
Fall

49
34

872
54

754

1675(±356)

1511(±272)
1406(±379)

1967

1457-

1 17°

1968

Spring
Fall

512

65
468

213
443

443
119

721 (±130)

1542(±306)

1969

Spring
Fall

339

245
1192

226
525

537
220

1008(±328)
2276(±766)

1970

Spring
Fall

145

142
1196

388
456

429

217

959(±161)
2014(±452)

1971

Spring
Fall

109

207
163

230
755

437
947

874(±151)
1974(±449)

"

1972

Spring
Fall

108

70
167

221
438

643
553

934 (±144)
1266 (±287)

1973

Spring
Fall

41

331

738

931

2041 (±457)

1974

Spring
Fall

644

34
411

167
881

645
980

846 (±129)
2916(±726)

1975

Spring
Fall

-

406

121

503

1030(±146)

1976

Spring

-

503

666

512

1681 (±371)

47

and the estimated biomass for any group of brown trout were not demon-
strated. The biomass of age I brown trout did explain 83 and 79%,
respectively, of the annual variation in the biomass of age II and
age III brown trout in succeeding years.

TABLE 12. Estimated biomass (lbs/6455 ft) of brown trout in the 1965
through 1974 year classes in the Hildreth Section of the
Beaverhead River.

Fall

Spring

Spring

Year Class

Agi

s Group I

Age Group

II

Age

Group III

1965

49

54

213

1966

34

65

226

1967

512

245

388

1968

339

142

230

1969

145

207

221

1970

109

70

-

1971

108

-

167

1972

41

34

121

1973

644

406

666

1974

-

503

Rainbow Trout

Spring and fall biomass estimates (lbs/6455 ft) of rainbow trout
by age groups are given in Table 13. Fall biomass estimates of age I
and older rainbow trout are compared in Figure 9. The biomass increased
slightly between 1966 and 1973, increased substantially between 1972 and
1974, then decreased slightly in 1975. The elevated biomass in 1974 and
1975 primarily reflect strong 1973 and 1974 year classes.

The biomass of the 1965 through 1973 year classes are shown for a two
year period in Table 14. Significant relationships between the magnitude
of mean flows during the irrigation and nonirrigation seasons and the
estimated biomass of any group of rainbow trout were not demonstrated.
However, the biomass of age I rainbow trout did explain 83% of the annual
variation in the biomass of age II rainbow trout the following fall.

48

TABLE 13. Estimated biomass (lbs/6455 ft) of rainbow trout by age groups
in the Hildreth Section of the Beaverhead River between 1966
and 1976. 80% confidence interval in parenthesis.

Age Group

I

II

III

IV & Older

Total

1966

Fall

80

144

224(±55)

240(±106)
351 (±122)

1967

Spring
Fall

22

240
329

1968

Spring
Fall

109

13
31

143
80

197
101

353(±158)
321 (±89)

1969

Spring
Fall

78

158

176

23
105

358
78

539(±159)
437(±103)

1970

Spring
Fall

54

32

99

199
173

223

49

454 (±98)
375 (±97)

1971

Spring
Fall

9

46
61

242
270

454
171

742 (±208)
511 (±206)

1972

Spring
Fall

146

0

59
33

779
301

838(±253)
480(±177)

1973

Spring
Fall

164

306

113

320

903(±251)

1974

Spring
Fall

1189

163
321

335
270

512
77

1010(±267)

1857(±379)

1975

Spring
Fall

721

871
676

314
84

405
23

1590(±190)
1504(±352)

1976

Spring

-

905

1235

614

2754(±630)

£

49

TABLE 14. Estimated biomass (lbs/6455) of rainbow trout in the 1965
through 1974 year classes in the Hildreth Section of the
Beaverhead River.

Fall

Fall

Fall

Year Class

Aqe Group I

Agi

s Group

II

Aqe

Group III

1965

80

.

80

1966

22

31

105

1967

109

176

173

1968

78

99

270

1969

54

61

33

1970

9

0

113

1971

146

306

270

1972

164

321

84

1973

1189

676

-

1974

731

-

-

Annual Rates

of Population Change

(Hi

ildi

-eth Section)

Annual spring to spring rates of population change of brown trout

by age groups are given in Table 15. Annual population increases probably

reflect the immigration of younger age groups following poor yearling

crops. Significant relationships between the annual rates of change for

any age group of brown trout and the magnitude of mean nonirrigation

flows were not demonstrated by simple regression analyses. The elevated

rate of population decrease of older brown trouL between 1967 and 1968

reflects water quality problems in addition to reduced flows.

TABLE 15. Annual rates of population change (%) of brown trout by age

groups in the Hildreth Section of the Beaverhead River between
1967 and 1976.

Brown

Trout

Aqe

Group

Spring to Spring

1 1 - 1 1 1

II

& Older

III

& Older-

III

& Older

IV

& Older

1967 - 68

+146

-60

-76

1968 - 69

+ 71

-24

-44

1969 - 70

- 14

-33

-49

1970 - 71

- 12

-40

-53

1971 - 72

- 49

-38

-30

1972 - 73

_

_

_

1973 - 74

_

_

_

1974 - 75

+ 91

-36

-50

1975 - 76

- 10

-17

-32

50

Annual fall to fall rates of population change of rainbow trout by

age groups are given in Table 16. The annual rate of population change

of age III and older rainbow trout was significantly correlated with

the magnitude of mean flows during the nonirrigation season (Figure 12).

Mean nonirrigation flows explain 66% of the annual variation in the rate

of decrease of age III and older rainbow trout. The annual rates of

decrease for all age groups of rainbow trout were elevated during the

period (1974-75) containing the lowest nonirrigation flows of the study.

TABLE 16. Annual rates of population change (%) of rainbow trout by age
groups in the Hildreth Section of the Beaverhead River between
1966 and 1975.

Rainbow Trout Age Group
Fall to Fall I-II II-III I & Older- II & Older- III & Older-

II & Older III & Older IV & Older

-49
-78
-36
-42
-28
-81
-94

Tag returns by fishermen were consistently low throughout the study
(Table 17). Although the river was open to year-round fishing, most of
the tagged fish were caught between April and September. Angler tag
returns were not elevated when mean April - September flows were lowest,
suggesting excessive harvesting by anglers was not the major cause of
the elevated rates of population decrease of older trout in low water
years.

1966 - 67

-

-

-12

-

1967 - 68

- 18

-

-58

-65

1968 - 69

- 25

+93

-18

- 8

1969 - 70

- 49

-30

-50

-50

1970 - 71

- 55

+68

-15

+ 10

1971 - 72

-100

-44

-46

-42

1972 - 73

- 19

-

- 9

+ 9

1973 - 74

+ 5

-52

-36

-64

1974 - 75

- 72

-82

-74

-86

'',1

100i

3 3

80

60

40

20

.82

300

100 200 300 400

MEAN FLOW (CFS) NONI R RIGAT ION

500

FIGURE 12. Relationship between the annual rates of population decrease
of age III and older rainbow trout and mean nonirrigation
flows (cfs) in the Hildreth Section of the Beaverhead River.

(%:

a/ Indicates significance at 5% level

52

TABLE 17. Relationship between tag returns of brown and rainbow trout by anglers

and mean April through September flows (cfs) for the Hildreth Section of
the Beaverhead River. Number of tag returns in parenthesis.

Period

% Return

1/

Mean Flow
April - September

1967, March - 1968, March

1968, March - 1968, October

1970, March - 1971, March

1973, October - 1974, October

1974, March -1974, December
1972, March -1973, March

1971, March -1972, March

7.7 (28)
2.2 ( 4)

6.5 (23)
5.0 (17)
4.0 (16)

13.6 (35)

5.6 (29)

312

481
647
679
679
749
847

■>, o, _ No. of angler tag returns during period ,nn
-' h No. of new fish tagged at start of period x luu

TABLE 18. Mean weights (lbs) by age groups of brown and rainbow trout captured by
electrofishing in the Hildreth Section of the Beaverhead River during
the 1966 through 1976 sampling periods.

Brown Trout

Age

Group

Spring

II

III

IV & Older

1967

1.01

_

_

1968

1.01

1.60

2.55

1969

1.03

2.04

3.14

1970

.94

1.89

2.98

1971

.87

1.73

2.65

1972

.94

1.82

3.07

1973

-

-

_

1974

1.06

1.87

3.30

1975

.87

1.98

3.55

1976

.81

1.59

3.68

Rainbow Trout

Age

Groups

Fall

I

II

III

IV & Older

1966

1.10

.

1967

1.29

-

-

-

1968

1.35

2.30

4.33

6.12

1969

.97

2.88

3.92

4.45

1970

.97

2.43

4.06

4.71

1971

.93

2.47

3.93

5.01

1972

1.05

-

2.37

5.02

1973

1.20

2.68

4.06

6.09

1974

1.19

2.24

4.94

5.23

1975

.91

2.40

3.26

5.64

53

Growth of Trout (Hildreth Section)

Mean weights by age group of brown trout in the spring and rainbow
trout in the fall are given in Table 18. Mean weights of all age groups
are exceptional compared to most other rivers in Montana. The growth of
trout in the Big Horn River, another tailwater fishery, is similar to
that in the Hildreth Section of the Beaverhead River (Stevenson, 1975).
Mean weights of age II and older brown trout in the fall and age II and
older rainbow trout in the spring are considered biased due to the move-
ment of nonresident spawning fish into the Hildreth Section.

Since growth-flow relationships for any age group of trout could
not be demonstrated, the influence of densities on growth was investigated
Mean weights of age II brown trout in the spring appear density related
(Figure 13). The estimated numbers of age II brown trout explain 66%
of the annual variation in the mean weights of age II brown trout.
Density-growth relationships were not demonstrated for any other age
group of trout.

Trout-Flow Relationships in All Sections

During the Spring of 1975 and 1976, numbers and biomass of trout
were estimated in a series of sections in the Beaverhead River (Appendix
Tables 19 and 20). In the spring of 1976, two sections, Grant (0.1)
and Pipeorgan Extended (8), were added. Estimates in 1975 were made
between March 6 and April 14 and in 1976 between March 10 and May 20.
Except for Grant (0.1), brown trout was the dominant species of trout
in the sections. The cause of the resurgence of rainbow trout numbers
in the Wheat (27) and Westside Canal (29) Sections is unknown. Spring
creeks in this area may be providing spawning habitat and recruits.

54

t too-

100

200 300 400 500

ESTIMATED NO OF AGE II BROWN TROUT

700

FIGURE 13. Relationship between the estimated numbers (N/6455 ft) and the
mean weights (lbs) of age II brown trout during the spring in
the Hildreth Section of the Beaverhead River.

a/ Indicates significance at 5% level.

55

Sections were selected primarily on the basis of differences in annual

flow regimes due to irrigation withdrawals and accretion. A comparison

of the minimum and maximum average daily flows in the nine study sections

provides a measure of the variations in flow that occur between sections

and between years (Table 21). The severe dewatering of the Grant (22 cfs

on October 16, 1974), Barretts Downstream (0 cfs on August 10, 1973),

and Anderson Lane (21 cfs on July 14, 1973) Sections has undoubtedly

affected aquatic life.

TABLE 21. Minimum and maximum average daily flows (cfs) between April 1
and March 31 in 9 sections of the Beaverhead River in 1973-74,
1974-75, and 1975-76.

Section 1973-74 1974-75 1975-76

Grant (0.1) 92-1070 22-1020 56-1320

Hildreth (2) 127-1115 57-1065 101-1365

Pipeorgan (8) 122-1110 52-1060 96-1360

Barretts Up. (15) 172-1150 110-1210 157-1480

Barretts Down. (18) 0- 746 62- 711 157- 933

Wheat (27) 148- 701 165- 702 238-1049

Westside Canal (29) 70- 701 90- 702 201- 974

Anderson Lane (38) 21- 725 100- 703 163- 997

Blaine (52) 75- 800 107- 772 153-1600

Comparison of trout populations in 1975 and 1976

The flows in the Beaverhead River in 1974-75, which preceeded the
Spring, 1975 population estimates, were considerably lower than those
which preceeded the Spring, 1976 estimates (Table 21). Between 1974-75
and 1975-76 minimum average daily flows in the nine study sections in-
creased by 43-155% and maximum average daily flows increased by 22-107%.
Trout populations in all nine sections would be expected to increase
following these dramatic flow increases if flow was the major limiting
factor throughout the system.

56

Population estimates in 1975 and 1976 were considered significantly
different if 80% confidence intervals did not overlap. Of these estimates,
numbers of age II and older brown trout in the Hildreth Extended (2),
Barretts Upstream Extended (15), and Barretts Downstream (18) Sections
increased significantly between years. Numbers of age II and older brown
trout within each of the lower four sections were not significantly
different between years. Numbers of age II and older rainbow trout in
the Hildreth Extended (2) Section were significantly greater in 1976,
although this increase partially reflects a greater movement of spawning
fish rather than an absolute increase in residents. Numbers of rainbow
trout within the Wheat (27) and Westside Canal (29) Sections were not
significantly different between years.

The increase in the estimated numbers of age II and older brown trout
between years in the Hildreth Extended (2), Barretts Upstream Extended
(15), and Barretts Downstream (18) Sections may partially reflect low
annual rates of population decrease resulting from greater flows between
the 1975 and 1976 estimates (Table 22). Although flows in the lower
river also increased dramatically, annual rates of population decrease
in the Wheat (27), Westside Canal (29) and Anderson Lane (38) Sections
were more than two times that in the other sections, suggesting factors
other than flow were limiting the survival of brown trout in these sections.

Between 1975 and 1976, the biomass of age II and older brown trout
increased significantly in the Hildreth Extended (2) and Barretts
Downstream (18) Sections, was not significantly different in the
Barretts Upstream Extended (15) and Blaine (52) Sections, and decreased
significantly in the Wheat (27), Westside Canal (29), and Anderson Lane

57

(38) Sections. Growth;in addition to numbers of trout, influenced the

differences in biomass between years. Mean weights at capture, as

indicated by age II brown trout, were lower in all sections in Spring,

1976 (Figure 14). Greater densities of trout in 1976 may explain the

lower mean weights in the Hildreth Extended (2), Barretts Upstream

Extended (15) and Barretts Downstream (18) Sections. However, densities

were similar in the lower four sections in both years. The consistency

of the weight decreases throughout the river would suggest that the

exceptionally high flows occurring between Spring, 1975 and 1976 retarded

growth. High flows may affect growth by lowering both water temperatures

and the availability of food.

TABLE 22. Relationship between annual rates of population decrease {%)
of age II and older brown trout and mean flows (cfs) in a
series of study sections in the Beaverhead River between
spring, 1975 and spring, 1976. Range of average daily flows
in parenthesis.

Sections

Decrease {%)
II & Older-
Ill & Older

Mean Flow

(cfs)

Spring, 1975-

Spring, 1976

Mean Flow

(cfs)
Irrigation

Mean Flow
(cfs) Non-
Irrigation

Hildreth
Extended (2)

26

624
(101-1365)

792

439

Barretts Up.
Extended (15)

30

702
(144-1480)

956

462

Barretts
Down. (18)

27

535

(144- 933)

613

460

Wheat (27)

61

624
(226-1049)

678

572

Westside
Canal (29)

66

592
(201- 974)

622

567

Anderson
Lane (38)

71

634

(163- 997)

622

644

Blaine (52)

30

749
(153-1600)

717

775

53

x 50

o

• SPRING, 1975
O SPRING1976

30

40

50

DISTANCE (MILES) BELOW DAM

FIGURE 14. Mean weights (lbs) of age II brown trout in a series of study
sections in the Beaverhead River in spring, 1975 and spring,
1976.

59

Only the Hildreth Extended (2) and Barretts Downstream (18) Sections
showed significant increases in both the numbers and biomass of brown
trout between 1975 and 1976. Biomass actually decreased significantly
in three of the study sections. The results presented earlier (Hildreth
Section) show that the flow increases that occurred between the 1975 and
1976 estimates were not the primary factor causing the population in-
crease in the Hildreth Extended (2) Section. It appears that the volume
of flow is not the only factor limiting trout populations in much of
the Beaverhead River.
Comparison of trout populations between sections

Differences in the estimated numbers and biomass of trout between
study sections (Figures 15 and 16) reflect a myriad of limiting factors
not common to all sections nor of the same magnitude. In addition to
flow differences, sedimentation and cover are factors of major concern.
Although the effects of sedimentation were not quantified in this study,
the decline in numbers and biomass of trout between the upper and lower
river is believed to partially reflect increasing sedimentation.
Sediment transport in the Beaverhead system is hindered by the diversion
of reservoir releases for irrigation. Removal of large amounts of water
reduce water velocities in the river to a point where the sediments can
no longer remain suspended and, therefore, are deposited in the lower
river. Mean turbidity measurements show that levels of suspended
sediments were lowest in the upper river (Table 23). Grasshopper Creek,
a major contributor of sediments to the Beaverhead River, is primarily
responsible for the increase in mean turbidity between miles 8 and 18.

en

FIGURE 15. Estimated numbers (N/1000 ft) of age II and older brown,
rainbow, and total trout in a series of study sections in
the Beaverhead River in spring, 1975 and spring, 1976.

61

FIGURE 16. Estimated biomass (lbs/1000 ft) of age II and older brown,
rainbow, and total trout in a series of study sections in
the Beaverhead River in spring, 1975 and spring, 1976.

62

TABLE 23. Summary of turbidity (JTU) measurements for 10 sites on the
Beaverhead River. Samples were collected between March 13,
1975 and February 27, 1976.

Approximate

Distance Below

No. Samples

Turbid

ty

(JTU)

Dam (miles)

Range

Mean

1.9

20

0.0- 7.0

2.0

2

20

0.8- 42.0

4.1

8

21

0.7- 32.0

4.5

18

21

0.7-110.0

11.1

24

21

0.8-110.0

12.6

30

21

0.8-110.0

12.0

30.5

20

0.8-108.0

13.5

38

21

0.9- 92.0

12.0

52

20

1.0- 54.0

13.5

67

21

1.2- 45.0

14.0

Cover plays an important role in limiting numbers of trout in the
upper river. The spring estimates of numbers of age II and older trout
in the Hildreth Extended (2) through Barretts Downstream (18) Sections
in both 1975 and 1976 (Figure 15) followed the same general trend as
willow bank cover (Table 1). Both decreased between the Hildreth (2)
Extended and Barretts Upstream Extended (15) Sections, then increased
substantially in the Barretts Downstream (18) Section. In the Wheat
(27) through Blaine (52) Sections, willow bank cover and numbers of
trout did not display similar trends, suggesting cover was not a major
factor influencing the decline in numbers of trout between these
sections.

The low number of trout in the Grant (0.1) Section in Spring, 1976
(Figure 15) may partially reflect inferior cover. Compared to the
Hildreth Extended (2) Section, willow bank cover in the Grant (0.1)
Section was rated as poor. However, the low population of trout may
also reflect flow reductions and water quality. Average daily flows

,

,

63

as low as 22 cfs have occurred in the Grant (0.1) Section in recent years
(October 16, 1974). Fish kills, caused by toxic levels of hydrogen
sulfide in reservoir releases, were documented in the Grant (0.1)
Section at the beginning of this study. Ammonia concentrations were also
elevated (see page 19). The severity and frequency of water quality
problems in the Grant (0.1) Section throughout the study period are
unknown.

Water quality problems also exist in other sections of the Beaver-
head River. Sampling by the Montana Department of Fish and Game and the
U.S. Food and Drug Administration in 1970-71 indicated that mercury
pollution occurred in the vicinity of Dillon. Some fish in this area
contained mercury greatly in excess of FDA standards. Natural spring
discharges, seed-potato treatment, the leaching of old mill tailings,
and possible industrial activity were considered the sources of mercury.
Above Grasshopper Creek, the Beaverhead River did not appear to contain
elevated levels of mercury.

Growth, in addition to numbers of trout, influenced the differences
in biomass between sections. A measure of the differences in growth
rates between study sections is provided by the mean weights at capture
of age II brown trout in Spring, 1975 and 1976 (Figure 14). Mean weights
decreased dramatically between the Grant (0.1) and Barretts Downstream
(18) Sections. The growth differences in the upper river are partially
attributable to Clark Canyon Reservoir, which has reduced diel fluctuations
in water temperature and has probably provided temperatures more favorable
for trout growth. However, the influence of the dam on water temperatures
decreased rapidly in approximately the upper 15 miles of river.

64

Food supply also influences the growth differences shown in Figure 14.

Smith (1973) found a substantial increase in zooplankton immediately below

the dam and speculated that this increase may provide additional food for

small fish through the first 6 miles of river below the reservoir.

Invertebrates also appear more abundant in the upper river. Limited kick

sampling in October, 1975 showed a greater abundance of invertebrates in

the riffles of the Hildreth (2), Pipeorgan (8), and Barretts Upstream (15)

Sections (Table 24). Tipulid larvae primarily accounted for the greater

volume of invertebrates in the Hildreth (2) and Barretts Upstream (15)

Sections.

TABLE 24. Summed results for kick samples collected in a series of study
sections in the Beaverhead River on October 22, 1975. Three
samples were collected in a riffle in each section.

No. Invertebrates Vol . Invertebrates
Section Collected Collected (ml)

Hildreth (2) 144 20.3

Pipeorgan (8) 179 6.8

Barretts Up. (15) 311 23.4

Barretts Down. (18) 39 2.6

Wheat (27) 44 1.3

Westside Canal (29) 118 3.7

Anderson Lane (38) 137 4.8

Blaine (52) 86 1.4

Comparison of trout populations throughout the study

All estimates of numbers and biomass of trout for the Pipeorgan (8),
Barretts Upstream (15), Wheat (27), and Blaine (52) Sections throughout
this study are given in Appendix Tables 25 through 29. It is doubtful
estimates in the Wheat (27) and Blaine (52) Sections primarily reflect
annual variations in flow. The consistently low estimates of numbers and
biomass of total and younger brown trout in the Blaine (52) Section
throughout this study suggest that reproduction is a problem. Reproductive

65

success is probably limited by the heavy deposition of sediments in spawning
gravels. Too few estimates are available in the Barretts Upstream (15)
Section to evaluate the impact of flows.

Spring estimates of age II and older brown trout in the Hildreth
(2) and Pipeorgan (8) Sections are compared in Table 30. Except for 1972,
total numbers exhibited similar trends in both sections. Fall estimates
of age I brown trout also displayed similar trends (Table 31), although
numbers of yearlings in 1967 and 1971 were considerably greater in the
Pipeorgan (8) Section. The similarity of population trends in sections
having similar flow regimes suggests flow was the dominant factor in-
fluencing numbers of trout in both the Hildreth (2) and Pipeorgan (8)
Sections.

Physical and Hydraulic Characteristics
Physical and hydraulic values at various flows were predicted for
20 cross sections in the Hildreth Section of the Beaverhead River using the
WSP (Water Surface Profile) computer program of the Bureau of Reclamation.
Each cross section was classified as a pool, run, or riffle. Mean
physical and hydraulic values were calculated for each water type. The
mean values at various flows in percent of the mean values at 1000 cfs,
the approximate bank full flow, are given in Appendix Table 32.

Flow reductions affected riffle areas in the Hildreth Section more
severely than pools and runs. Five of the six measured characteristics
showed the greatest rates of loss in riffles (Appendix Table 32).
Riffles are important to trout because they are the primary food pro-
ducing and spawning areas in a stream. The importance of desirable
flow patterns in riffle areas during the rainbow and brown trout
spawning periods was demonstrated during this study.

66
TABLE 30. Comparison of the spring estimates of numbers (N/1000 ft) of age II
and older brown trout in the Pipeorgan (8513 ft) and Hildreth
(6455 ft) Sections of the Beaverhead River between 1968 and 1976.
80% confidence interval in parenthesis.

Brown Trout
N/1000 ft

Spring Pipeorgan Hildreth

1968 96 (±23) 58 (±11)

1970 122 (±16) 77 (±11)

1971 138 (±37) 84 (±15)

1972 166 (±21) 62 (± 8)
1974 74 (±14) 49 (± 8)
1976 171 (±36) 184 (±44)

TABLE 31. Comparison of the fall estimates of numbers (N/1000 ft) of age I
brown trout in the Pipeorgan (8513 ft) and Hildreth (6455 ft)
Sections of the Beaverhead River between 1967 and 1973.

Fall

1967
1968
1969
1970
1971
1972
1973

Brown
N/1000

Trout
ft

Pipeorgan

Hi

Idreth

24

6

88

73

83

68

34

25

72

22

33

25

8

9

67

Top width and wetted perimeter were proportionately less affected by
changes in discharge than were the other characteristics in pools, runs
and riffles of the Hildreth Section. In pools, mean velocity and cross
sectional area were most affected by changes in discharge, followed in
order by mean depth and maximum depth. In runs, cross sectional area
was most affected, followed in order by mean velocity, mean depth, and
maximum depth. The order for riffles was cross sectional area, mean
depth, maximum depth, and mean velocity.

Mean physical and hydraulic values in percent of the mean values at
1000 cfs for all 20 cross sections in the Hildreth Section are compared
in Figure 17. Mean values for the various characteristics increase as
streamflow increases, but the rate of gain becomes less as the volume
of flow fills the river channel. Visual inspection of Figure 17 shows
that the rate of loss for all of the measured characteristics was greatly
accelerated at flows less than approximately 200 cfs. For all water
types combined, cross sectional area was most affected by changes in
discharge, while top width and wetted perimeter were least affected.

Cross sectional area, an index of the total amount of living space
available for fish, was greatly affected by changes in discharge in all
water types. Based on the mean values at bank full flow (1000 cfs),
mean cross sectional area in pools, runs, and riffles in the Hildreth
Section is reduced by 50% at flows of approximately 275, 311, and 380
cfs, respectively (Appendix Table 32). Average daily flows as low as
57 cfs (October 16, 1974) were recorded in the Hildreth Section during
this study. At this flow, mean cross sectional area in pools, runs,
and riffles was approximately 25, 18, and 14%, respectively, of the
mean values at bank full flow.

GH

FIGURE 17. Mean values of physical and hydraulic characteristics at various
flows (cfs) in percent of the mean values at 1000 cfs for 20 cross
sections in the Hildreth Section of the Beaverhead River.

69

Values of the six physical and hydraulic characteristics at various
flows were predicted for a total of 66 cross sections in 7 other study
sections in the Beaverhead River. Based on the values at 1000 cfs,
changes in the six characteristics with respect to discharge were
generally similar to those trends which occurred in pools, runs, and
riffles of the Hildreth Section. Appendix Table 33 lists those flows
yielding a 50% reduction in the measured characteristics based on the
mean values at 1000 cfs. Again, cross sectional area, an index of the
total amount of living space available for fish, was greatly affected
by changes in discharge. Based on 1000 cfs, flows of approximately 340
cfs throughout the Beaverhead River for all water types combined would
yield an approximate 50% reduction in the amount of living space avail-
able for fish.

DISCUSSION

The presence of large numbers of trophy size trout characterizes
the quality fishery of the upper Beaverhead River. Brown and rainbow
trout as large as 12.0 and 13.25 lbs, respectively, were captured by
electrofishing during this study. Numbers of trophy trout (> 5.0 lbs)
were not estimated due to small sample size. However, numbers of
trout > 5.0 lbs captured during each of the sampling periods between
1966 and 1976 provide an index of the abundance of trophy fish (Table
34). Trophy brown trout were not captured until the last five years of
the study. Changes in numbers captured did not appear to relate to flows
Trophy rainbow trout were captured throughout the study. Numbers
captured, which were greatest in 1973 and 1974, crashed in 1975. This
dramatic reduction followed the lowest mean nonirrigation flow (97 cfs)
of the study.

70

TABLE 34. Numbers of brown and rainbow trout>5.0 lbs captured by electro-
fishing in the Hildreth Section (6455 ft) of the Beaverhead River
between 1966 and 1976.

Spring

Number
Captured

Brown Trout s 5.0 lbs
No. Electrofishing
Runs

No. Captured/Run

1967

0

1968

0

1969

0

1970

0

1971

0

1972

3

1973

-

1974

10

1975

13

1976

4

0
0
0
0
0
0.8

2.5
2.6
1.0

Rainbow Trout ^5.0 lbs

Number

No.

El

ectrofishi

ng

Fall

Captured

Runs

No.

Captured/Run

1966

3

2

1.5

1967

2

2

1.0

1968

11

3

3.7

1969

5

3

1.7

1970

8

4

2.0

1971

11

4

2.8

1972

9

4

2.3

1973

16

3

5.3

1974

13

3

4.3

1975

1

4

0.3

71

The survival of the larger and older rainbow trout in the Hildreth
Section of the Beaverhead River is directly related to the magnitude of
flows during the nonirrigation season (approximately October 15 - April
15). During this period, Clark Canyon Reservoir is storing water for
irrigation and releases into the Beaverhead River are minimal. A measure
of the absolute minimal average daily flow that will maintain adequate
survival and numbers of older rainbow trout is provided by the distrib-
ution of average daily flows between successive Fall estimates (Appendix
Table 35). The three Fall to Fall periods having the lowest rates of
population decrease of age III and older rainbow trout and the greatest
numbers of age IV and older rainbow trout remaining at the end of each
period are 1970-71, 1971-72, and 1972-73. Compared to the other Fall to
Fall periods, 1970-71, 1971-72, and 1972-73 have fewer average daily
flows in the 101-150 cfs interval and none in the 57-100 cfs interval.
The data suggests that one average daily flow <_ 150 cfs is sufficient
to adversely affect the annual survival of older rainbow trout (Figure
18).

Few rivers in Montana are capable of producing a trophy rainbow
trout fishery equal to the potential of the upper Beaverhead River.
The ability of the upper Beaverhead to produce trophy rainbow trout
is dependent on flow releases from Clark Canyon Reservoir. Appendix
Table 35 provides an index of the minimum average daily flow that will
maintain desirable numbers of trophy rainbow trout. The highest estimate
of numbers of age IV and older rainbow trout followed a period (1971-72)
in which only 6 of the average daily flows were <_ 250 cfs and 7 were
£ 300 cfs. It appears that the management of the Hildreth Section of

72

00

«r-

i —

QJ

r0

i , ^

+J

S-

vJ 1 ou

ro

+->

S-

s_

oo ro

QJ

3 E

T3

O T-

03

.— 4->

Z5

O

H- 00

C

OJ

C

"O

>■>

ro

c

i — c

ro

•r- O

CD

ro ■!-

_C

-O 4->

+J

i — i

(13

OJ i—

c

CD 3

QJ

QJ

re O.

QJ

en

S- o

3

ro

QJ O-

-s -

QJ 4- r0 r—
JD O I—

4-> QJ C QJ

ro S- O

i— U QJ <_>

CH "O +-> 00

SdO 091 > SMCHd A1IVQ 30VU3AV dO U39WnN

i

73

the upper Beaverhead River as a high quality, trophy rainbow trout fishery
will require average daily flows greater than approximately 250 cfs.
This corresponds to a flow release of approximately 200 cfs at Clark
Canyon Dam.

Positive relationships between the magnitude of flows during the
nonirrigation season and the survival and numbers of older brown trout
were not demonstrated during this study. However, the period (1971-72)
producing the lowest rate of population decrease of age III and older
brown trout and having the highest number of age IV and older brown
trout at the end of the period has the fewest average daily flows in
the 101-300 cfs interval (Appendix Table 34).

The annual rates of population decrease of older rainbow trout
during low flow periods were greater than those of older brown trout.
During the period containing the lowest nonirrigation flows of the study
(1974-75), age III and older rainbow trout showed a 94% rate of decrease,
while that of age III and older brown trout was only 50%. The relative
stability of the estimates of numbers of age IV and older brown trout
and the instability of estimates of age IV and elder rainbow trout
throughout this study primarily reflect a differential rate of decrease
during low flow periods.

Habitat preferences may explain the differential rates of population
decrease documented in this study. Although the components of the pre-
ferred habitat of brown and rainbow trout are poorly understood, some
difference have been measured. Lewis (1969) found cover (primarily
overhanging vegetation, undercut banks, and brush) to be the most
important factor influencing numbers of brown trout in stream pools,
while current velocity was most important for rainbow trout. Kraft

74

(1968) measured the rate of change of cover (overhanging vegetation and
undercut banks) and current velocity with respect to flows. He concluded
that cover was not greatly reduced by high levels of dewatering, while
current velocity showed the greatest rate of change of the physical
parameters measured. When flows are reduced, the preferred habitat of
older rainbow trout in the Hildreth Section may be diminishing at a
faster rate than that of older brown trout. This may account for the
elevated rates of population decrease of older rainbow trout during low
flow periods.

Poor reproductive success was the major factor limiting the total
numbers and biomass of trout in the Hildreth (2) Section. Reproductive
success throughout much of this study was inadequate for providing suf-
ficient numbers of trout to fill all available living space. It is
doubtful populations of brown trout of age groups I, II and III and
rainbow trout of age groups I and II reflect the carrying capacity of
the Hildreth Section, even during low water years. The simultaneous
increases in the estimated total numbers and biomass of brown and
rainbow trout, which occurred in the Hildreth Section during the last two
years of the study, demonstrate the inadequacy of reproductive success
in previous years. These increases, which coincide with some of the
lowest mean nonirrigation flows of the study, were the result of two
strong year classes (1973 and 1974) in succession. With flow releases
favorable to both reproduction and the survival of older trout, the
Hildreth Section is capable of supporting greater numbers and biomass
of trout of all age groups than those which existed throughout much of
this study.

/'.

The biological and water surface profile data collected in this study
would support the management of the upper Beaverhead River (Hildreth
Section) for a minimum average daily flow of approximately 250 cfs. An
evaluation of the water surface profile data suggests the 250 cfs
minimum would apply to the lower river as well. However, other factors
adversely affecting fish populations in the lower river may minimize
any benefits gained by increased flows during the irrigation and non-
irrigation seasons. Limited biological data collected during this study
does suggest that factors other than flow are contributing to the
decline in numbers and biomass of trout that occurs between the upper
and lower river.

76

LITERATURE CITED

Dooley, J. M. 1976. Application of U. S. Bureau of Reclamation Water
Surface Profile Program (WSP). Pages 478-495 in. Proceedings of
the Symposium and Specialty Conference on Instream Flow Needs, Vol.
II. Amer. Fish. Soc, 5410 Grosvenor Lane, Bethesda, Maryland.

Kraft, M. E. 1968. The effects of controlled dewatering on a trout
stream. M. S. Thesis, Montana State Univ., 31 pp.

Lewis, S. L. 1969. Physical factors influencing fish populations in
pools of a trout stream. Trans. Amer. Fish. Soc. 98:14-19.

Montana Department of Fish and Game. 1975. Beaverhead River and Clark
Canyon Reservoir Fishery Study. Montana Department of Fish and
Game. 62 pp.

Needam, R. G. and A. H. Wipperman. 1967. Inventory of the waters of
the project area. Job Completion Rep., Fed. Aid Proj . F-9-R-14,
Job No. I. Montana Dept. of Fish and Game. 20 pp.

Smith, K. M. 1973. Some effects of Clark Canyon Reservoir on the

limnology of the Beaverhead River in Montana. M. S. Thesis, Montana
State Univ., 62 pp. (Unpublished).

Spence, L. E. 1975. Guidelines for using Water Surface Profile Program
to determine instream flow needs for aquatic life. Preliminary
Draft Rep. Montana Dept. Fish & Game, Env. & Info. Div., Helena.
33 pp.

Stevenson, H. R. 1975. The trout fishery of the Big Horn River below
Yellowtail Dam, Montana. M. S. Thesis, Montana State Univ., 67 pp.
(Unpublished) .

Vincent, E. R. 1971. River electrofishing and fish population estimates
Prof. Fish Cult. 33(3): 163-169.

Wipperman, A. H. 1965. Lake and reservoir investigations. Job Com-
pletion Rep. Fed. Aid Proj. F-9-R-13, Job No. V. Montana Dept.
of Fish and Game. 14 pp.

Wipperman, A. H. and A. Elser. 1968. Inventory of the waters of the
project area. Job Completion Rep., Fed. Aid Proj. F-9-R-16, Job
No. I. Montana Dept. of Fish and Game. 14 pp.

APPENDIX

78

TABLE 5. Correlation coefficients for the multiple linear relationships between
the spring estimates of numbers of age IV and older brown trout and
mean irrigation and nonirrigation flows in the Hildreth Section of the
Beaverhead River.

Dependent Variables

Partial Correlation Coefficients

No. Ill & Older
Brown Trout
Previous Spring

Mean Flow
Non-
irrigation

Multiple

Correlation

(r)

No. IV & Older Brown Trout

,4616

,4618

,5266

Partial Correlation Coefficients

No. Ill & Older

Brown Trout Mean Flow

Previous Spring Irrigation

Multiple
Correlation

No. IV & Older Brown Trout

.5196

.4537

.5200

79

2 "-

000-

•

800>

r =

a

-.84

600-

•

•

400-

200-

• •

•

•

LOG VARIANCE

FIGURE 8. Relationship between the log variance of the average daily flows
(cfs) during the brown trout spawning period (September 15 -
October 31 ) and the estimated number of age I brown trout two
years later in the Hildreth Section of the Beaverhead River.

a/ Indicates significance at 5% level.

80

TABLE 8. Correlation coefficients for the multiple linear relationships
between fall estimates of numbers of age III and age IV and
older rainbow trout and mean irrigation and nonirrigation flows
in the Hildreth Section of the Beaverhead River.

<

Partial Correlation Coefficients

Dependent Variables

No. of age II
Rainbow Trout
Previous Fall

Mean Flow Multiple
Nonirrigation Correlation (r)

No. of age III
Rainbow Trout

.2432

.1614

2603

No. of age II
Rainbow Trout
Previous Fall

Mean Flow
Irrigation

Multiple
Correlation (r)

No. of age III
Rainbow Trout

.1599

,2876

.3493

No. of age IV &
Older Rainbow Trout

No. of age III & Older

Rainbow Trout Mean Flow Multiple

Previous Fall Nonirrigation Correlation (r) I

77S9-7

.8534

1/

.9019-/

No. of age III & Older

Rainbow Trout Mean Flow

Previous Fall Irrigation

Multiple
Correlation (r)

No. of age IV &
Older Rainbow Trout

5577

.0184

5598

1/ Indicates significance at 5% level

81

CM

,-

CO

CSJ

<SJ

o

oi

CO

CO

8

CO

Ot

£

00

£

o

•

o

"!

^

o

to

CO

to

-

r— a>

m to

CO o

i— at

o .—

o ■!->

o c

r— 0)

>• i—

co at to

to to to

oo —

o LTJ o

to <d- CM

to .—

I- -O <-> T3

ai t-> i/> —

8?

o5 QJ

> I—

c r-.

CT>

o0 0)

«— ^O

C\J «tf"

CM CSJ

CO «3" r—

CO ro CO

r— O <T>

83

TABLE 25. Estimated numbers (N/1000 ft) and biomass (lbs/1000 ft) of brown
trout by age groups in the Pipeorgan Section (8513 ft) of the
Beaverhead River between 1967 and 1976. Rainbow trout were too
scarce to estimate. 80% confidence interval in parenthesis.

Brown Trout

N/1000 Ft Lbs/1000 Ft

IV & IV &
I II III Older Total I II III Older Total

1967 Oct. 24 59 83 (±22) 136.5 136.5 (±38.4)

1968 Apr. -- 51 24 21 96 (±23) 143.2 143.2 (±37.9

Oct. 88 60 27 175 (±39) 64.0 111.7 74.4— 250.1 (±69.4

1969 Oct. 83 96 18 <1 197 (±34) 52.6 167.5 47.8 .7 268.6 (±51.8

1970 Mar. -- 64 54 4 122 (±16) 47.2 83.8 9.9 140.9 (±17.7

Oct. 34 108 33 175 (±27) 22.6 174.9 80.4— 277.9 (±51.5

1971 Mar. — 32 74 32 138 (±37) 24.3 94.0 56.3 174.6 (±40.4

Oct. 72 35 70 177 (±29) 36.3 37.8 152.3— 226.4 (±41.0

1972 Mar. — 72 43 51 166 (±21) 45.9 54.8 103.4 204.1 (±30.0

Sept. 33 47 26 29 135 (±28) 19.9 79.0 58.6 90.2 247.7 (±64.2

1973 Oct. 8 32 28 14 82 (±14) 4.8 53.0 75.0 49.3 182.1 (±34.3

1974 Mar. — 9 37 28 74 (±14) — 7.0 62.2 65.7 134.9 (±26.1
1976 Mar. — 91 80 171 (±36) — 62.9 122.3— 185.2 (±35.6

84

TABLE 26. Estimated numbers (N/1000 ft) and biomass (lbs/1000 ft) of brown
trout by age groups in the Barretts Upstream Section (5792 ft)
of the Beaverhead River between 1967 and 1976. Rainbow trout
were too scarce to estimate. 80% confidence interval in
parenthesis.

Brown Trout

N/1000 Ft

IV &
I II III Older Total

Lbs/1000 Ft

IV &
II III Older Total

1967

Mar

Oct

1975

Mar

1976

Mar

31 41 72 (±13)

30 29 24 83 (±25)

83 28 12 123 (± 8)

94 93 187 (±40)

29.0 — 69.3 98.3 (±16.7)

25.4 49.1 55.5 130.0 (±43.0)

52.6 38.5 23.2 114.3 (± 7.8)

46.4 — 111.1 157.5 (±36.6)

TABLE 27. Estimated numbers (N/1000 ft) and biomass (lbs/1000 ft) of brown

trout by age groups in the Wheat Section (9908 ft) of the Beaverhead
River between 1968 and 1976. 80% confidence interval in parenthesis.

Brown Trout

N/1000 Ft

IV &
I II III Older Total

Lbs/1000 Ft

i.

II

IV &
III Older

Total

1 3UO

1971

UUII .

Jul.

■ji

45

25

26

1972

Jul.

34

41

23

1973

Jul.

22

1974 Oct. 128 26

1975 Mar.

1976 Mar.

81

138 (±30) 16.9

61

— 106.3 123.2 (±23.0)

8 104 (±25) 10.8 24.5 37.5 15.7 88.5 (±22.9)

8 106 (±16) 10.7 36.8 31.4 15.3 94.2 (±13.3)

83 (±19) 7.3 77.3 84.6 (±19.8)

41

16 211 (±43) 70.7 36.2 73.9 38.1 218.9 (±36.4)

- 88 — 60

- 110 —57

148 (±15)
167 (±36)

56.7 92.3 149.0 (±15.4)

40.3 65.3 105.6 (±17.5)

85

TABLE 28 Estimated numbers (N/1000 ft) and biomass (lbs/1000 ft) of rainbow
I trout by age groups in the Wheat Section (9908 ft) of the Beaverhead

River between 1968 and 1976. 80% confidence interval in parenthesis,

Jun.
Jul.

Rainbow

Trout

Lbs/K

N/1

000 Ft

)00 Ft

I

II

III

IV &
Older

Total

I

II

III

IV &

Older Total

1968
1971

— No Estinr

— No Estirr

1972

Jul.

—

21

20

7

48 (±12)

20.0

29.2

11.5 60.7 (±15.5)

1973

Jul.

28

28 (±11)

41.8

41.8 (±16.2)

1974

Oct.

26

18

16

13

73 (±17)

15.4

26.4

35.7

36.7 114.2 (±26.8)

1975

Mar.

—

32

23 —

55 (±10)

22.6

44

.4 — 67.0 (±12.3)

1976

Mar.

--

28

53 —

81 (±27)

11.7

60

.1 — 71.8 (±23.5)

TABLE 29. Estimated numbers (N/1000 ft) and biomass (lbs/1000 ft) of brown

trout by age groups in the Blaine Section (17837 ft) of the Beaverhead
River between 1971 and 1976. Rainbow trout were too scarce to
estimate. 80% confidence interval in parenthesis.

Brown Trout

N/1000 Ft Lbs/1000 Ft

IV & IV &
I II III Older Total I II III Older Total

1971 Aug. --9 9 3 21 (± 6) — 8.4 13.3 8.3 30.0 (±7.0)

1972 Sept. 9 10 10 4 33 (± 5) 3.9 11.3 15.9 9.1 40.2 (±6.4)

1975 Apr. — 14 —26 40 (± 4) — 7.5 34.1 41.6 (±3.8)

1976 May -- 22 —28 50 (±11) — 9.5 36.2 45.7 (±10.9)

1L_

86

TABLE 32. Mean values of hydraulic characteristics at various flows (cfs) in

percent of the mean values at 1000 cfs for pools, runs, and riffles in
the Hildreth Section of the Beaverhead River.

Pools

Flow

Mean

Max.

Mean

Top

Cross Sect.

Wetted

(cfs)

Vel.

Depth

Depth

Width

Area

Perimeter

50

21

48

46

67

24

66

75

26

51

49

73

28

71

100

31

54

50

77

31

75

150

38

58

53

85

37

83

175

42

60

55

85

40

85

200

45

62

55

86

43

84

250

50

66

59

87

48

85

300

55

69

62

89

52

87

400

64

74

68

90

61

89

500

72

80

77

92

68

91

600

79

84

83

94

75

93

750

88

90

91

96
Runs

84

95

50

29

35

32

56

16

53

75

32

42

37

65

23

63

100

36

46

41

72

27

75

150

44

52

47

78

33

78

175

47

54

50

81

37

83

200

50

56

50

83

39

84

250

56

61

53

85

45

86

300

62

66

55

86

49

87

400

68

72

65

89

58

91

500

75

76

69

92

66

93

600

81

82

76

93

74

96

750

89

89

88

95
Riffles

84

97

50

41

30

26

56

13

54

75

50

33

29

59

15

56

100

54

37

32

64

18

61

150

60

43

39

74

25

73

175

62

46

43

77

28

73

200

64

49

43

79

31

77

250

71

54

45

81

35

79

300

71

58

48

86

42

83

400

11

66

58

88

52

86

500

82

72

66

90

61

89

600

86

78

74

91

69

91

750

92

87

87

95

81

93

87

TABLE 33. Approximate flows (cfs) at which various hydraulic characteristics are reduced
by 50% in relation to the mean values at 1000 cfs in pools, runs, and riffles
' in a series of study sections in the Beaverhead River.

Sections
POOLS

No. Cross

Mean

Max.

Mean

Top

Cross Sect.

Wetted

Sections

Vel.

Depth

Depth

Width

Area

Perimeter

Hildreth

7

250

75

100

<50

275

<50

Pipeorgan

5

250

75

150

<25

240

<25

Barretts Up.

2

<25

250

300

75

400

75

Barretts D.

3

250

75

75

38

250

38

Wheat

1

125

225

200

25

350

38

Wests ide C.

3

300

<25

<25

100

188

88

Anderson L.

3

175

125

150

38

360

38

Blaine

1

300

38

<25

<25

188

<25

RUNS

Hildreth

5

200

150

200

<50

300

<50

Pipeorgan

4

125

200

175

25

360

25

Barretts Up.

1

150

225

250

<25

340

<25

Barretts D.

1

150

175

300

25

340

38

Wheat

6

163

125

163

<25

300

<25

^ Westside C.

3

300

<25

<25

50

200

50

Anderson L.

5

75

238

275

50

400

50

Blaine

3

200

175

113

<25

350

<25

RIFFLES

Hildreth

8

75

200

320

<50

380

<50

Pipeorgan

4

100

188

240

88

380

63

Barretts Up.

4

38

300

450

50

500

50

Barretts D.

4

88

250

300

<25

400

25

Wheat

2

88

350

400

<25

450

<25

Westside C.

4

100

213

225

38

380

25

Anderson L.

0

—

—

-_

__

Blaine

7

75

300

325

<25

450

<25

TOTAL

Hildreth

20

150

150

200

<50

320

<50

Pipeorgan

13

175

150

200

<25

300

25

Barretts Up.

7

38

275

370

50

450

63

Barretts D.

8

150

175

210

25

325

25

Wheat

9

100

163

225

<25

350

<25

Westside C.

10

225

25

<25

50

275

50

Anderson L.

8

100

188

225

38

400

50

► Blaine

11

125

250

250

<25

400

<25

88

•

PART II

FISH POPULATIONS IN
CLARK CANYON RESERVOIR AND TRIBUTARIES

"

90

TABLE OF CONTENTS

Page

TABLE OF CONTENTS 90

LIST OF TABLES 91

LIST OF FIGURES 92

CONCLUSIONS 93

RECOMMENDATIONS 94

INTRODUCTION 95

DESCRIPTION 95

Physical Reservoir Statistics 95

Limnology 97

METHODS 99

RESULTS 102

Reservoir Fish Populations 102

White and Longnose Suckers 102

Brown Trout 107

Burbot 107

Other Species 107

Tributary Fish Populations 107

Horse Prairie Creek 107

Red Rock River 110

Fishery (1964-1976) 112

Growth of Hatchery Rainbow Trout 112

Fishing Pressure 114

LITERATURE CITED 117

91

LIST OF TABLES

TABLE 1. Morphometric data for Clark Canyon Reservoir at the

average operating level--elevation 1688.6 m (5540 ft) .

TABLE 2. Fish caught in three gill net sets, Clark Canyon

Reservoir, August, 1964

TABLE 3. Fish caught in seven gill net sets, one surface set

and six bottom sets Clark Canyon Reservoir, September,
1965

TABLE 4. Fish caught in six gill net sets, two surface sets and
four bottom sets Clark Canyon Reservoir, September,
1966

TABLE 5. Fish caught in 18 gill net sets, 9 surface sets and 9
bottom sets, Clark Canyon Reservoir, June, 1968 . . . .

TABLE 6. Fish caught in three gill net sets, Clark Canyon

Reservoir, June, 1969

TABLE 7. Fish caught in 21 trap net sets, Clark Canyon Reservoir,
June, 1969. Nets set selectively to catch burbot . . .

TABLE 8. Fish caught in eight trap net sets, Clark Canyon

Reservoir, May, 1970

TABLE 9. Fish caught in two gill net sets, Clark Canyon

Reservoir, May, 1971

TABLE 10. Fish caught in eight gill net sets, Cl^rk Canyon

Reservoir, July, 1972

TABLE 11. Estimated trout populations for a 7590 ft section of

Horse Prairie Creek located two miles upstream of Clark
Canyon Reservoir. 80% confidence limit in parenthesis.

TABLE 12. Estimated trout populations for a 5500 ft section of
the Red Rock River in August, 1969. 80% confidence
limit in parenthesis (from Peterson, 1971)

TABLE 13. Estimated trout populations for a 6250 ft section of

the Red Rock River in June, 1974. 80% confidence limit
in parenthesis (from Peterson, 1975)

TABLE 14. Number and size of rainbow trout stocked in Clark

Canyon Reservoir, 1964 to 1976 (from Montana Department
of Fish and Game, 1975)

Page

95

103

103

103
104
104
104
105
105
105

109

111

111

113

92

LIST OF FIGURES

FIGURE 1. Contour map of Clark Canyon Reservoir

FIGURE 2. Maximum and minimum storage pools in Clark Canyon

Reservoir for the 1965 through 1976 water years. . .

FIGURE 3. Seasonal abundance of the five major algal genera

in Clark Canyon Reservoir (from Berg, 1974)

FIGURE 4. Average length of suckers caught by gill nets and
trap nets, Clark Canyon Reservoir, 1964-72 (From
Montana Department of Fish and Game, 1975)

FIGURE 5. Average length of brown trout and burbot caught by
gill nets and trap nets, Clark Canyon Reservoir,
1964-72 (from Montana Department of Fish and Game,
1975)

FIGURE 6. Growth of hatchery rainbow trout. Clark Canyon

Reservoir, 1970, 71 and 72 (from Montana Department
of Fish and Game, 1975)

Page
96

100

106

108

115

93

CONCLUSIONS
Clark Canyon Reservoir develops a thermocline, turning over in spring
and fall. The reservoir can be classified as a fertile environment.
White sucker was the most numerous fish species netted in the reservoir
between 1964 and 1972, comprising from 40 to 93% of the net catches.
Hatchery rainbow trout comprised over 90% of the annual catch of
game species by anglers. An estimated 41,000 and 39,000 rainbow
trout were harvested in 1971 and 1972, respectively. Brown trout and
burbot were also taken.

Few carp have been observed in the reservoir. Their impact on the
trout fishery appears negligible.

Reservoir populations of white and longnose suckers use Horse Prairie
Creek for spawning.

Conflict between resident trout populations in Clark Canyon trib-
utaries and spawning populations of suckers could not be determined
from limited biological data collected in this study.
Preimpoundment populations of trout in the Red Rock River appear to
be greater than those after impoundment. However, this may reflect
habitat differences rather than factors related to impoundment.
Growth of hatchery rainbow trout during their first year in the
reservoir is dependent on planting time.

Estimates of fishing pressure in fisherman days were 30,400 in
1971, 36,000 in 1972, and 40,587 in 1975-76 (May-April). Fishing
pressure increased by approximately 34% over a 6-year period.

94

RECOMMENDATIONS

1. The optimum storage pool necessary to maintain a desirable reservoir
fishery was not determined in this study. Additional information should be
gathered during years of abnormal fluctuation.

2. Hatchery rainbow trout should be planted in the reservoir annually
in the early spring just after the reservoir turnover, 3 to 7 weeks after
ice-out.

95

INTRODUCTION
The Montana Department of Fish and Game has monitored fish populations
in Clark Canyon Reservoir and its tributaries since 1964. The purpose of the
study was to determine the relative abundance of game and nongame fish within
the reservoir and to determine the influence of rough fish populations on
resident fish 1n tributaries to the reservoir.

DESCRIPTION
Physical Reservoir Statistics
Clark Canyon Reservoir (Figure 1) was constructed on the Beaverhead
River in southwestern Montana during the period 1961 to 1964. The primary
functions of the reservoir are irrigation and flood control, The drainage
area for the reservoir encompasses 2321 square miles. At maximum water
surface elevation (5571.9 ft), the reservoir has a surface area of 6600
acres and a storage capacity of 328,979 acre feet. Morphometric charac-
teristics of the reservoir at the average operating level of 5540 ft are
given in Table 1. The maximum depth of 93.5 ft is located approximately

TABLE 1. Morphometric data for Clark Canyon Reservoir at the average operating
level—elevation 1688.6 m (5540 ft).

Maximum Depth

Mean Depth

Maximum Length

Maximum Breadth

Mean Breadth

Area

Volume

Length of Shoreline

Shoreline Development

28.5 m (93.5 ft)

9.35 m (30.68 ft)

6.47 km ( 4.02 mi)

5.01 km ( 3.11 mi)

3.06 km ( 1.90 mi)

19.789 x 106 m2 (4.890 x 103 acres)

18 x 106 m3 ( 150 x 103 acre-ft)

26.01 km (16.16 mi)

1.65

96

Clark Canyon Reservoir
Beaverhead County, Montana

Scale
4000 Feet

i 4000 reel i

I 1219 Meters I

Figure 1. Contour map of Clark Canyon Reservoir

97

600 ft from the dam. Since the inverted outlet is only 8.5 ft above the
old streambed, discharge into the Beaverhead River comes from the deepest
part of the reservoir. Generally, the reservoir is completely ice covered
by mid-December and ice-out occurs by mid-April.

The maximum and minimum storage pools in the reservoir for the 1965
through 1976 water years are compared in Figure 2. The severe drawdown
in the summer of 1975 is the result of basing water allotments on predicted
inflows from watershed snowpack information, rather than the known content
of the reservoir. This procedure is no longer practiced.

Limnology

Limnological data was taken from a study conducted by Berg (1974)
between June, 1971 and November, 1972.

Clark Canyon Reservoir characteristically develops a thermocline,
turning over in spring and fall. Thermocline depth ranged from 6.6 to
73.8 ft and euphotic zone depth ranged from 16.4 to 50.9 ft. The minimum
and maximum conductivity readings were 420 and 650 uohms, respectively.
Dissolved oxygen near the surface ranged from 6.5 to 10.8 ppm and from
0.2 to 11.1 ppm on the bottom. Low DO concentrations in water over 50
ft in depth limit the use of this portion of the reservoir by fish. The
pH ranged from 7.8 to 8.6 and, as with DO, highs occurred during overturn
and lows during summer stagnation. Following spring and summer overturn,
levels of plant nutrients in the euphotic zone were reduced, probably as
a result of uptake by phytoplankton and transfer to the deep water zone
by sinking. Ranges of total alkalinity and total hardness were 177 to
222 ppm as CaCO., and 157 to 230 ppm as CaCO^, respectively.

98

240 T

r* 200-

oc 180
O

160

140

120

2 100-

o

w 80

60-

• MAXIMUM
o MINIMUM

■o— — o-

65 66 67 68 69 70 71 72 73 74 75 76

WATER YEAR

Figure 2. Maximum and minimum storage pools in Clark Canyon
Reservoir for the 1965 through 1976 water years.

99

A calcium ion concentration of 2.99 me/1 was measured in a euphotic
zone water sample during the fall overturn, 1972. According to Reid's
(1961) classification for biological productivity, a calcium ion concen-
tration greater than 1.25 me/1 is considered "rich". Clark Canyon con-
tains over twice this amount and must be considered a fertile environ-
ment.

From December, 1971 to November, 1972, thirty-one algal genera (6
classes) were observed in the euphotic zone of Clark Canyon Reservoir.
In order of decreasing abundance, the five genera with the largest mean
annual standing crops were: Asterionella, Aphanizomenon, Cryptomonas,
Rhodomonas, and Synedra. The seasonal abundance of these genera are
compared in Figure 3. A blue-green algae bloom, composed primarily of
Aphanizomenon sp. , peaked in September, 1972. Water temperature was the
only variable measured that was significantly correlated with phytoplank-
ton standing crops.

Five major zooplankton taxon were encountered during the study.
Daphnia schodleri and Cyclops bicuspidatus thomasii were the dominant
zooplankters during all seasons. D. schodleri became the first abundant
zooplankter, appearing in large numbers in mid-May. Water temperatures
and chlorophyll ^concentrations were the only measured variables signif-
icantly correlated with populations of _D. schodleri.

METHODS
Fish were sampled in Clark Canyon Reservoir using gill nets, trap nets,
and seine nets. The gill nets used were 125-foot x 6 foot experimental nets

100

a

o

k_

o

O)

c
c

<0

*•>
(0

c
o

*■>

c

a
o

a

c

0

ra

c

F

O
E
o

0

,2

E

ra

c

0

n

ra

tl

0)

0

r

c

>.

<1

u

oc

in

0)

a;

o

m

a

cfl

F

TD

o

c

'-

3

4-

X)

cn a)

to c/)

a) a>

to Pi

101

with five 25-foot panels of different mesh size: 3/4, 1, 1^, 1% and 2-inch.

Trap nets were 4 x 7-foot frames covered with 1-inch mesh net; leads were
approximately 25 yards. The nylon seine was 100 x 12 ft.

A percentage of the hatchery rainbow trout stocked in Clark Canyon
were marked for future identification using fin clips. Clips used were:
adipose-- 1970, left pelvic-- 1971, and adipose— 1972. Rainbow trout in
creels of reservoir fishermen were checked for these marks and also were
identified as wild or hatchery by the appearance and shape of the fins.
Hatchery fish had fins with bent rays and rounded tail fins.

Fish from Red Rock River and Horse Prairie Creek were sampled using

boat electrofishing gear as described by Vincent (1971). Fish population

estimates were made using the mark and recapture method as expressed by

(M+l) (C+l) where M is the number of fish marked, C is the number of
(R+l)

fish recaptured, and R is the number of marked fish recaptured. Population
estimation calculations were done on a hand calculator prior to 1969. The
1969-1972 fish population calculations were done on a computer using the
most recent Montana F1sh and Game computer format.

Fishing pressure on Clark Canyon Reservoir was determined in 1971
using pressure curves as outlined by Peterson (1970). In 1972, fishing
pressure was determined using the method of Neuhold and Lu (1957). This
method is based on a stratified random sample. The year was stratified
into 4-week periods starting March 15 and further stratified into week-
days and weekend days. Two weekdays per week were selected at random with
subsequent weekdays in 'the following 3-week period selected such that each

102

weekend was sampled at least once per 4-week period. The weekdays were
thus sampled 8 days out of 20 per period. Every other weekend day was
sampled. The first weekend day per sampling period was selected at
random, then ewery other weekend day was sampled. Holidays were treated
as weekend days. Each selected sample day was divided into four 4-hour
periods. On each sample day, two counts were made, one in one 4-hour
period and one in a different period. The 4-hour periods were selected
at random while the hours within periods were in sequential order after
the first random selection.

Catch was determined by contacting fishermen and recording number,
species and size of fish in the creel, plus the number of hours required
to take the fish.

RESULTS
Reservoir Fish Populations

Gill and trap nets were used to census fish populations in Clark Canyon
Reservoir from 1964 through 1972. Catch statistics for each year are given
in Tables 2 through 10. White sucker (Catostomus commersoni) was the domi-
nant species in the catch in all years. Other species captured were long-
nose sucker (Catostomus catostomus), burbot (Lota lota), mountain whitefish
(Prosopium williamsoni), brown trout (Salmo trutta), rainbow trout (Salmo
gairdneri), brook trout (Salvelinus fontinalis), and carp (Cyprinus carpio).

White and Longnose Suckers

Average lengths of white and longnose suckers at capture are compared
in Figure 4. The average length of both species remained fairly constant
between 1964 and 1968. Between 1968 and 1969, both species attained the
greatest annual increase in average length during the study period. After

103

T\BLZ 2. Fish caught in three gill net sets, Clark Canyon Reservoir, August, 1QM

Number

Percent of

Size range

Ave

. length

Ave

. weight

Species

caught

catch

in inches

in

inches

in

pounds

White Sucker

378

67.0

6.8-18.5

11.1

0.76

Longnose Sucker

108

19.1

6.6-19.8

10.9

0.78

Mountain Whitefish

29

5.1

8.5-15.8

14.2

1.39

Rainbow Trout

5

0.9

10.8-13.4

11.5

0.63

Brown Trout

39

6.9

8.0-23.3

12.3

1.04

Brook Trout

5

0.9

9.7-15.8

12.3

0.90

Taken from Wipperman, 1965,

TABLE 3. Fish caught in seven gill net sets, one surface set and six bottom sets
Clark Canyon Reservoir, September, 1965

Number Percent of Size range Ave. length Ave. weight
Species caught catch in inches in inches in pounds

White Sucker

391

81.8

6.3-17.3

11.8

0.81

Longnose Sucker

20

4.2

9.4-18.9

12.2

0.85

Mountain Whitefish

31

6.5

8.0-17.4

12.0

0.88

Rainbow Trout

25

5.2

8.4-14.5

11.5

0.69

Brown Trout

8

1.7

11.0-23.3

18.1

2.81

Brook Trout

3

0.6

10.9-16.4

13.9

1.38

Taken from Needham and Wipperman, 1967

TABLE 4. Fish caught in six gill net sets, two surface sets and four bottom sets
Clark Canyon Reservoir, September, 1966.

Species

Number
caught

Percent of
catch

Size range
in inches

Ave. length
in inches

Ave. weight
in pounds

White Sucker

490

73.3

6.0-19.4

11.4

0.75

Longnose Sucker

114

17.1

6.7-19.5

11.7

0.74

Mountain Whitefish

31

4.6

9.5-18.3

14.5

1.46

Rainbow Trout

4

0.6

18.8-21.2

19.7

3.19

Brown Trout

22

3.3

15.1-22.0

18.8

2.90

Brook Trout

3

0.4

15.0-16.2

15.7

1.87

Burbot

4

0.6

7.9-16.2

11.6

0.46

Taken from Wipperman, 1967

104

TABLE 5. Fish caught in 18 gill net sets, 9 surface sets and 9 bottom sets,
Clark Canyon Reservoir, June, 1968.

Species

Number
caught

Percent of
catch

Size range
in inches

Ave. length
in inches

Ave. weight
in pounds

White Sucker

490

42.6

7.3-16.2

12.3

0.92

Longnose Sucker

311

27.0

7.4-19.5

11.6

0.68

Mountain Whitefish

71

6.2

11.5-19.6

15.8

1.97

Rainbow Trout

9

0.8

11. 0-21. 2

17.2

2.67

Brown Trout

61

5.3

6.2-24.3

19.4

3.30

Brook Trout

1

0.1

16.4

2.20

Burbot

207

18.0

8.0-22.8

14.2

0.85

Carp

1

0.1

7.8

0.32

Taken from Elser, 1969.

TABLE 6. Fish caught in three gill net sets, Clark Canyon Reservoir, June, 1969.

Number

Percent of

Size range

Ave

. length

Ave,

. weight

Species

caught

catch

in inches

in

inches

in

pounds

White Sucker

475

90.1

6.1-17.0

13.6

Longnose Sucker

13

2.5

11. 6-16.0

13.3

Mountain Whitefish

1

0.2

Rainbow Trout

1

0.2

13.0

Brown Trout

17

3.2

7.1-24.2

20.5

Brook Trout

1

0.2

Burbot

19

3.6

12.3-22.1

15.5

Unpublished data from Marcoux, 1969.

TABLE 7 . Fish caught in 21 trap net sets, Clark Canyon Reservoir, June, 1969.
Nets set selectively to catch burbot.

Number

Percent of

Size range

Ave

. length

Ave

. weight

Species

caught

catch

in inches

in

inches

in

pounds

White Sucker

1297

39.5

7.8-18.2

13.9

1.19

Longnose Sucker

373

11.3

9.9-21.3

15.8

1.50

Mountain Whitefish

20

0.5

8.2-19.2

16.2

Rainbow Trout

9

0.3

14.3-24.8

20.3

2.97

Brown Trout

15

0.5

19.5-23.9

21.7

3.12

Brook Trout

1

0.0

14.1

1.08

Burbot

1570

47.8

11.9-27.0

16.8

0.96

Carp

4

0.1

11.5-14.4

13.2

1.61

Unpublished data from Marcoux, 1969.

105

TABLE 8. Fish caught in eight trap net sets, Clark Canyon Reservoir, May, 1970.

Number Percent of Size range Ave. length Ave. weight
Species caught catch in inches in inches in pounds

White Sucker

1937

92.6

7.3-18.8

14.8

1.62

Longnose Sucker

47

2.2

13.2-19.4

15.9

1.67

Mountain Whitefish

6

0.3

11.9-18.2

16.1

1.50

Rainbow Trout

4

0.2

13.0-17.5

14.8

1.36

Brown Trout

8

0.4

19.2-23.1

20.6

2.53

Burbot

89

4.3

14.2-30.4

19.9

1.98

Taken from Peterson, 1971.

TABLE 9. Fish caught in two gill net sets, Clark Canyon Reservoir, May, 1971.

Number Percent of Size range Ave. length Ave. weight
Species caught catch in inches in inches in ponds

White Sucker
Longnose Sucker
Mountain Whitefish
Rainbow Trout
Brown Trout
Burbot

Taken from Montana Department of Fish and Game, 1975.

TABLE 10. Fish caught in eight gill net sets, Clark Canyon Reservoir, July, 1972,

30

46.9

9.3-16.8

14.2

3

4.7

6.8-12.3

10.5

1

1.6

16.8

23

35.9

11.4-18.4

13.9

6

9.4

17.1-22.5

19.3

1

1.6

15.6

Number

Percent of Size range Ave. length Ave. weight
bpecies caught catch in inches in inches in pounds

White Sucker

381

Longnose Sucker

37

Mountain Whitefish

0

Rainbow Trout

41

Brown Trout

8

Burbot

21

78.1

8.8-17.6

7.6

7.6-18.3

0.0

8.4

7.2-21.6

1.6

15.7-21.6

4.3

13.2-17.6

15.2
15.4

14.2
14.2
15.6

Taken from Montana Department of Fish and Game, 1975.

106

I7r

16

ul5

14

5 13

> I2h

?

Inadequate sample in 1971

© © longnose sucker

x x white sucker

J L

64 65 66 67 68 69 70 71 72
year

FIGURE 4 . Average length of suckers caught by gill nets and trap
nets, Clark Canyon Reservoir, 1964-72 (From Montana
Department of Fish and Game, 1975),

107

1970, average weights were relatively stable.

The average length at each annulus for white suckers were as follows:
age I - 1.5 inches; II - 5.4 inches; III - 10.6 inches; IV - 13.4 inches;
V - 14.7 inches; VI - 16.3 inches; and VII - 17.0 inches (Peterson, 1971).
Suckers do not appear in the net catch in significant numbers until they
are at least 3 years old.
Brown Trout

Brown trout were first captured in 1964. Between 1964 and 1969 mean
lengths increased, peaked in 1969, then decreased between 1969 and 1972
(Figure 5).
Burbot

Burbot were first captured in 1966. Mean lengths increased between
1966 and 1970, peaked in 1970, then decreased between 1970 and 1972
(Figure 5).
Other Species

A few brook trout were netted through 1969, after which no brook trout
were captured. Mountain whitefish never comprised more than 6.5% of the
catch. No whitefish were captured in 1972. A few carp were captured in
1968 and 1969. Seven small carp, 3.8 to 4.4 inches in length, were taken by
seining at night in 1970'. Clark Canyon Reservoir does not support a pro-
lific population of carp.

Tributary Fish Populations
Horse Prairie Creek

Trout population estimates for a section of Horse Prairie Creek, located
approximately two miles upstream of Clark Canyon Reservoir, are given in

21-
B

.£ 16

en 14-

c

w 12

>

t

108

/ \

&> •©

.©

x x brown trout

© © burbot

64 65 66 67 68 69 70 71 72
year

%

FIGURE 5. Average length of brown trout and burbot caught by gill
nets and trap nets, Clark Canyon Reservoir, 1964-72
(from Montana Department of Fish and Game, 1975).

109

Table 11. Estimates of numbers and biomass of total trout decreased

42 and 28%, respectively, between 1970 and 1972. However, the biomass

decrease was not significant on the basis of overlapping 80% confidence

intervals. The number and biomass of rainbow trout showed the greatest

decrease, 73 and 67%, respectively. Water withdrawals for irrigation have

affected this stream in the past and may be partially responsible for these

reductions.

TABLE 11. Estimated trout populations for a 7590 ft section of Horse Prairie
Creek located two miles upstream of Clark Canyon Reservoir. 80%
confidence limit in parenthesis.

July, 1970 May, 1972

Species Size group (in ) Number Pounds Number Pounds

Brown Trout > 6.0 345 365 230 294

(±60) (±84) (±30) (±38)

Rainbow Trout > 7.0 100 79 27 26

(±35) (±27) (± 9) (± 9)

TOTAL 445 444 257 320

(±69) (±88) (±31) (±39)

During the summer of 1969, about 1800 white suckers and 400 longnose
suckers greater than 7.8 inches were tagged in Clark Canyon Reservoir. In
the Spring of 1972, 6 tagged white suckers were recovered in Horse Prairie
Creek, approximately two miles upstream of the reservoir. In May of 1972,
in the same location in Horse Prairie Creek, 2813 white and 421 longnose
suckers were marked. During July, two marked suckers were recovered in
the reservoir, showing movement out of Horse Prairie Creek following spawning,

In one week in approximately 3000 feet of Horse Prairie Creek, 1,387
white and 345 longnose suckers were captured of which 70 white and 3 long-
nose suckers were marked. In a small stream such as this where a high

no

efficiency would be expected, the small number of recaptures indicates a
migrating population.
Red Rock River

A trout population estimate for a 5500 ft section of the Red Rock
River, located approximately two miles upstream of the reservoir, was made
in August, 1969 (Table 12). Total numbers and biomass of age 1 and older
trout were 182/1000 ft and 135 lbs/1000 ft, respectively. Brown trout com-
prised 91 and 92% of the estimated number and biomass of trout, respectively.
Mountain whitefish and longnose suckers were abundant, but were not
quantitatively sampled. Tagging results indicate trout movement does occur.
Within one year, two tagged trout from this section were caught in the
reservoir, another had moved through the reservoir and was caught seven
miles downstream in the Beaverhead River, and one was captured ten miles
upstream from the section.

A trout population estimate for a 6250 ft section of the Red Rock River,
located several miles upstream of the section sampled in 1969, was made
in June, 1974 (Table 13). Brown trout comprised 85 and 79% of the estimated
number and biomass of trout, respectively. The total number and biomass
of trout/1000 ft were 135 and 124%, respectively, of those estimated in
the downstream section in 1969.

In 1961, a total of 128 rainbow and brown trout greater than 7 inches
and weighing 135 pounds were captured by electrofishing in 659 ft of the
Red Rock River, now inundated by the reservoir. This is 197 and 208 lbs/1000
ft by number and weight, respectively. An actual estimate would be higher,
since it is unlikely that all trout in the section were captured. Consider-

Ill

TABLE 12. Estimated trout populations for a 5500 ft section of the
Red Rock River in August, 1969. 80% confidence limit in
parenthesis (from Peterson, 1971).

Species

Age group

Number/1000 ft Pounds/1000 ft

Brown Trout

Total
Rainbow Trout

Total
Total Brown and Rainbow Trout

I

II

III

IV

V and older

79
46
28
11
2

19.9
29.4
47.4
23.4
4.9

166
(±32)

125.0
(±24)

I

II

III

IV and older

7
5
3

1

1.5
2.9
3.6
2.3

16
(±5)

10.3
(±3)

182
(±32)

135
(±24)

TABLE 13. Estimated trout populations for a 6250 ft section of the
Red Rock River in June, 1974. 80% confidence limit in
parenthesis (from Peterson, 1975),

Species

Age or size group

Number/1000 ft Pounds/1000 ft

Brown Trout
Rainbow Trout

I and older

> 8.7 in

(mostly II and older)

209(±23)
36(± 8)

132(±12)
36(± 8)

Total Brown and Rainbow Trout

245(±24)

168(±14]

112

ing these conservative totals, the number of trout was comparable to the
number estimated for the two upstream sections, while weight was at least
24 and 54% greater than the biomass estimated for the two upstream sections,

Fishery (1964-1976)

Since Clark Canyon Reservoir began impounding water in 1964, the
Montana Department of Fish and Game has annually stocked rainbow trout.
Trout have been planted in varying numbers and sizes and at different
times of the year (Table 14). Creel census information during 1967,
1968, 1970 and 1973-76 showed over 90% of the catch to be rainbow trout,
of which 90-95% were of hatchery origin. Brown trout was the other trout
species taken.
Growth of Hatchery Rainbow Trout

Characteristic of most newly created reservoirs, trout growth was
exceptional immediately following impoundment. One and one-half years
after introduction, the 3-5 inch fingerlings planted in October, 1964
averaged greater than 17.5 inches. After being in the reservoir 27 months,
these fish averaged 21.0 inches in length. Hatchery rainbow were marked
and their growth monitored by creel census from 1970 through 1975. Trout
growth was not as great during this period as in 1965 and 1966. Twelve
months following introduction, the trout planted in 1970, 1971 and 1972
averaged about 15.0 inches (Figure 6). After two years, these rainbow
trout averaged between 18.5 and 19.0 inches. Only a few known age trout
were creeled after three years and these averaged 21.0 inches.

113

TABLE 14. Number and size of rainbow trout stocked in Clark Canyon
Reservoir, 1964 to 1976 (from Montana Department of Fish
and Game, 1975).

Time
interval

Total

number

planted

Fish size
in inches

Percent
in size

10/1/64 - 10/16/64

180,085

3
4
5

30
50
20

3/9/65'

351,700

3

100

1/18/66

262,126

3

100

9/13/66

44,000

3-4

100

4/17/67 - 4/26/67

170,460

4

100

9/20/67 - 9/26/67

83,340

4

100

4/29/68 - 5/6/68

165,465

4

100

6/18/69 - 7/23/69

128,140

6

100

6/4/70 - 8/6/70

151,556

6

7

77
23

5/4/71 - 7/6/71

144,657

5
6

25
75

4/4/72 - 5/30/72

252,920

5
6

84
16

5/7/73 - 6/11/73

256,104

5
5

39
61

4/9/74 - 4/17/74

150,000

5

100

4/28/75 - 5/14/75

198,535

5
6

67
33

4/19/76 - 5/13/76

210,586

5
6

18
82

114

Growth during the first year in the reservoir appears to be directly
related to the planting time (Figure 6). The trout planted in April and
May (1972) displayed greater growth than those planted in June, July or
August (1970, 1971). Seven months following introduction, the trout planted
in April and May, 1975 averaged 14.3 inches, about 4 inches longer than
rainbow trout planted for the same duration in 1970. During the second
year in the reservoir, the greatest variation between years is about 1.5
inches while during the third year there is very little difference.

During the study, storage in the reservoir varied greatly between years
(Figure 1). Growth of hatchery rainbow trout did not appear to be related
to either the maximum or minimum storage pools.
Fishing Pressure

In 1971 and 1972, an intensive creel census was conducted. Ice fishing
data were included with the data for bank fishing. Although some ice fisher-
men were fishing with the intent of catching burbot, trout were kept when
caught.

Fishing pressure in 1971 for bank and boat fishing was 53,241 and
60,201 fisherman hours, respectively, and for 1972, 67,631 and 67,939
fisherman hours, respectively. Total fishing pressure increased about 20%
between 1971 and 1972. Bank fishing increased 27% while boat fishing in-
creased 16%. The greatest increase in bank fishing occurred during the
weekdays (49%). The greatest increase in boat fishing occurred on the
weekends (16%)

The estimated number of fisherman days for 1971 was 30,400 and
36,000 for 1972. Most fishermen had not completed fishing when contacted.

115

tO <N — U 0> OD

saMDui ui H10N31 3AV

fJ

i/)

Q x

»-
z

2

CD

o en
> i—
S-

qj -
i/) CD
CD E

O TD

C ro

=s E
O +->
S_ i-

+-> ro

Q.

2 CD

O Q

_Q
C ro

•i- C
ro ro
S- +->

c
>, o

.£= E

o o

ro W-

116

Estimates of fisherman days are slightly elevated, since fisherman hours

were divided by the average hours fished in obtaining pressure in fisherman

days.

A statewide fishing pressure survey, conducted by mail during 1975-76,

indicated the following pressure in fisherman days for Clark Canyon

Reservoir:

Fisherman Days
Period (80% confidence interval)

May 1, 1975 - September 30, 1975 30,329 (+4347)

October 1, 1975 - April 30, 1976 10,258 ( + 4602)

40,587 (+ 6331)

Catch rates ranged from 0.25 to 0.45 in 1971 and 0.19 to 0.36 in
1972 for individual time periods. The catch rate/hour was greater for
boat fishermen for both 1971 and 1972.
Yield

An estimated 41,000 and 39,000 rainbow trout were harvested in 1971
and 1972, respectively. In 1971, 67% of the catch was taken during the
five month period between May 17 and October 17. In 1972, 93% of the
catch was taken during the seven month period between March 4 and October
15. During both years, the catch was comprised almost entirely of two
age groups of hatchery rainbow trout.

i

117

LITERATURE CITED
Berg, R. 1974. Limnology of Clark Canyon Reservoir, Montana. M.S. Thesis,

Montana State University, Bozeman, Montana. 79 pp.
Elser, A. A. 1969. Inventory of the waters of the project area, Southwest

Montana fishery study. Montana Dept. of Fish and Game, Project

F-9-R-17, Job I, June 11, 1969. 16 pp.
Marcoux, R. G. 1960. (Unpublished data). Gill and trap netting data on

Clark Canyon Reservoir. Present address: Montana Dept. of Fish and

Game, Bozeman, Montana.
Montana Department of Fish and Game, 1975. Beaverhead River and Clark

Canyon Reservoir Fishery Study. Montana Dept. of Fish and Game,

Bozeman, Montana. 62 pp.
Neuhold, J. M. and K. H. Lu. 1957. Creel census method. Utah State

Dept. of Fish and Game, Publication No. 8, Salt Lake City, Utah. 36 pp.
Peterson, N. W. 1970. The yield of wild and hatchery trout from Big

Spring Creek, Montana. M.S. Thesis, Montana State University,

Bozeman, Montana. 35 pp.
Peterson, N. W. 1971. 1970 summary of Clark Canyon-Beaverhead River

project. Montana Dept. of Fish and Game, Bozeman, Montana. 19 pp.
Peterson, N. W. 1975. Inventory of the waters of the project area,

Southwestern Montana fisheries study. Montana Dept. of Fish and

Game, Project F-9-R-23, Job I-b, July 15, 1975. 17 pp.
Reid, G. K. 1961. Ecology of inland waters and estuaries. Van Nostrand

Rienhold Company, New York. 375 pp.
Vincent, E. R. 1971. River electrofishing and fish population estimates.

Prog. Fish. Cult., 33(3) :163-169.

118

Wipperman, A. H. 1965. Lake and reservoir investigations, Southwest
Montana fishery study. Montana Dept. of Fish and Game, Project
F-9-R-13, Job V, February 11, 1965. 8 pp.

Wipperman, A. H. 1967. Inventory of the waters of the project area,
Southwest Montana fishery study. Montana Dept. of Fish and Game,
Project F-9-R-15, Job I, September 29, 1967. 14 pp.