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Full text of "Evaluation of excavated fish rearing pool in Vincent Creek"

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Filing Code 6763 
Date Issued 3/30/73 

Bureau of Land Management U.S. DEPARTMENT OF THE INTERIOR 


John W, Anderson, Fishery Biologist 
Coos Bay District, Oregon 


The streams of southwestern Oregon that flow through the tyee sandstone 
formation in the region often lack sufficient pool rearing area to 
support extensive populations of salmonids during low summer flows. A 
test rearing area was created in Vincent Creek to determine if such areas 
would provide habitat for salmonids during the late summer low water 
critical period. The results of this experiment appear favorable and 
applicable to other streams that lack sufficient rearing pool area . 
Several smaller pools were created during this work, but the experi- 
mental pool described shows the most promise. 


Vincent Creek was selected as a stream improvement pilot project since 
absence of rearing pools in the mainstem were a limiting factor. A 
ratio of 80 percent riffle to 20 percent pool was estimated after 
detailed stream survey. Most of the riffle area is less than three 
inches deep. Several natural self-cleaning pools were found to 
provide excellent fish habitat.. 

It was decided that the creation of pools would be the single most 
beneficial habitat improvement that could be economically undertaken. 
The use of small dams or log sills was considered and rejected for 
these structures could not provide a permanent pool area in bedrock 
and would require periodic maintenance. After studying the natural pool 
areas for more than a year, it was decided that construction of similar 
pools was desirable. Proper location was a necessity if the pools were 
to retain their full size. A small waterfall or plunge at the head of 
each pool appeared necessary to accomplish proper flushing. One natural 
pool with a two-foot plunge was completely filled by a landslide but 
cleaned itself of most debris after two winters of high flows during 


An experimental pool was excavated by blasting in the fall of 1970. A 
track-drill was used to make the powder holes and the material was shot 
free of the hole by using heavy charges coupled to a time delay sequence. 
(Illustration 1) The material shot free of the hole was loaded on 

Additional copies of Technical Notes ore available from DSC, Federal Center Building 50, Denver, Colo., 80225 

a dump truck with a front end loader; the rock was then used as road 
riprap. Inspection of the area revealed that the heavy charges blew 
away much of the vegetative canopy and barked surrounding trees. 

The work was accomplished through the use of a force account crew 
which makes it difficult to calculate the equivalent cost to excavate 
a similar hole through a bid contract. It is estimated that the test 
hole cost approximately $500-5600 . Larger 20-foot by 50-foot pools, 
which will be blasted for the future Vincent Creek pilot project, 
are estimated at this time to cost under $1500, 

The pool was created at the mouth of a small tributary in order to take 
advantage of the cold water supplied in warm weather. During summer 
low flow, Vincent Creek was approximately 50 feet wide in this area 
and about 2 to 3 inches deep over a solid sheet of bedrock, (Illustra- 
tion 5) The resulting pool was approximately 20 feet long and 25 feet 
wide. The upstream end was about 9 feet deep and downstream end 
4 to 5 feet deep, (Illustration 2) 

The pool has now been subjected to two winters of severe high water. 
A state of "natural repose" appears to have been reached at about 
M-,5 feet of depth. Some silted gravel is deposited around the less 
turbulent areas of the pool and these fluctuate l-l^g feet annually 
depending upon severity of freshets. Illustration 4 shows the general 
flow patterns that have developed in the pool. 

In Oregon and several other states working in a stream is subject to 
state law and also may require permission for underwater blasting. 


The test hole was blasted out of a flat sheet of bedrock which did not 
result in a high plunge at the head of the pool necessary for complete 
flushing. The gradient of the stream provided only a 3 to 6-inch 
plunge at the head of the pool. The test pool should have had at least 
a one-foot plunge to keep the pool flushed. Consequently, the pool has 
filled to a depth of H.5 feet as shown in Illustration 2. 

The 20-foot length of the test pool was too short to allow the water to 
slow down before it hit the back of the pool. The larger natural pools 
are about 50 feet long and generally have a small gravel deposit dropped 
by the slowing water. These deposits are used heavily by steelhead 
trout for spawning. The gravel and smaller materials laid down at the 
tail of the pool also provide environment for aquatic life of all types 
that are excellent fish food. 

In order to retain a natural appearance, it is advisable to use lighter 
charges and excavate the materials from the hole with a back hoe or 
clam shovel. This procedure would entail diverting the water around the 
work site during excavation. Objectionable drilling silt would be 

Illustration 1 

J *Drill set on 
a one foot 

9' 9 8' 8' 6' 4' Drill depth * 
6 5-j 5k 5h 4 2 Sticks 40% powder 


Silt Bar 

--^_-^ Hole 


Illustration 3. 

>— Bottom af 

Bottom after 
2 years 

ter blasting 


Illustration 2. 

Bottom afte; 

Bottom after 
2 winters 


Illustration 4. 


prevented from entering the stream and diversion provides a dry work 
area. Drilling silts from the work site are highly visible and must 
receive attention during the advanced planning stage to prevent 
objectionable reaction bv the public. 

The area was virtually devoid of fish life and most other aquatic life 
was in limited supply before creation of the pool. Sampling of the 
pool has shown an abundance of fish life and aquatic organisms. 
Several adult cutthroat trout were present. Juvenile cutthroat trout, 
steelhead trout and coho salmon were numerous. Blacknose dace and 
redside shiners were extremely abundant. Cottids and brook lamprey 
were present. Crayfish, periwinkles, caddis fly larva and many other 
aquatic organisms were abundant. It was estimated that the total fish 
population in the pool ranged from 200-400 and approximately 50 percent 
of the fish were salmonids. 


The test pool has been in existence for more than two years. It is 
considered successful at this time because: 

1. It provides habitat for all forms of aquatic life 
as well as an extensive fish population. 

2. It is self-cleaning and maintenance free. 

3. It is reasonably economical to construct. 

4. It is aesthetically and environmentally 
acceptable in that it looks completely natural 
two vears after creation. 

Illustration 5. 

Track drill and operator working during late summer low flow. The Alder tree in the 
upper right corner of the photo can be used as a reference mark when viewing the 
pool after creation as seen in illustration 6. 


Illustration 6. 

Photo of test pool taken after creation during a period of moderate winter flow. 
Note the slack water area on the far side of the pool where fen eddy has formed 
at the mouth of the tributary.