546 LIFE ON THE MISSISSIPPI.
Under the general principle that the local slope of a river is the resufc
and measure of the resistance of its bed, it is evident that a narrow and dee&
stream should have less slope, because it has less frictional surface in wo.
portion to capacity; ».<*., less perimeter in proportion to area of cross section.
The ultimate effect of levees and revetments confining the floods and bring,
ing all the stages of the river into register is to deepen the channel and let
down the slope. The first effect of the levees is to raise the surface 5 fcut
this, by inducing greater velocity of flow, inevitably causes an enlargement
of section, and if this enlargement is prevented from being made at the ex-
pense of the banks, the bottom must give way and the form of the waterway
be so improved as to admit this flow with less rise. The actual experience
with levees upon the Mississippi River, with no attempt to hold the banki,
has been favourable, and no one can doubt, upon the evidence furnished
in the reports of the commission, that if the earliest levees had "bees
accompanied by revetment of banks, and made complete, we should haro
to-day a river navigable at low water, and an adjacent country safe from
inundation.
Of course it would be illogical to conclude that the constrained river cm
ever lower its flood slope so as to make levees unnecessary, but it is believed
that, by this lateral constraint, the river as a conduit may be so improved
in form that even those rare floods which result from the coincident rising
of many tributaries will find vent without destroying levees of ordinaiy
height. That the actual capacity of a channel through alluvium depend
upon its service during floods has been often shown, but this capacity does
not include anomalous, but recurrent, floods.
It is hardly worth while to consider the projects for relieving the Mb*
sissippi River floods by creating new outlets, since these sensational pre-
positions have commended themselves only to unthinking minds, and hare
no support among engineers. Were the river bed east-iron, a resort to open-
ings for surplus waters might be a necessity; but as the bottom is yielding
and the best form of outlet is a single deep channel, as realising the least
ratio of perimeter to area of cross section, there could not well be a mm
•onphilosophical method of treatment than the multiplication of avenues of
escape. V
In the foregoing statement the attempt has been made to condense m
as limited a space as the importance of the subject would permit, tbs
general elements of the problem, and the general features of the propose^
method of improvement which has been adopted by the Mississippi
Commission*
The writer cannot help feeling that it is somewhat presumptuous on T
part to attempt to present the facts relating to an enterprise which calls
ine highest scientific «1nll; but it is a matter which interests every