Life On The Mississippi By Mark Twain




















































































































































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Under the general principle that the local slope of a river is the
result and measure of the resistance of - Page 510
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Under The General Principle That The Local Slope Of A River Is The Result And Measure Of The Resistance Of

Its bed, it is evident that a narrow and deep stream should have less slope, because it has less frictional

Surface in proportion to capacity; i.e., less perimeter in proportion to area of cross section. The ultimate effect of levees and revetments confining the floods and bringing 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; but this, by inducing greater velocity of flow, inevitably causes an enlargement of section, and if this enlargement is prevented from being made at the expense 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 banks, has been favorable, and no one can doubt, upon the evidence furnished in the reports of the commission, that if the earliest levees had been accompanied by revetment of banks, and made complete, we should have 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 can 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 ordinary height.

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