332 happ. STREAM-CHANNEL CONTROL [Ch. 18 



Regulation has been accomplished chiefly by permeable pile training 

 dikes, revetment to protect concave banks along the desired alignment, 

 rock-filled dams to close chutes, and dredging and dragline excavation 

 to hasten the adjustments and cut off a number of particularly sharp 

 or long bends. The first cutoff was made especially to equalize ab- 

 normal disparities in slope immediately above and below the mouth of 

 the Platte River in Nebraska, which has a much steeper slope and de- 

 livers more bed-load sand to the Missouri than the latter can readily 

 carry away (Neff, 1940). 



The permeable dikes, consisting of rows of clumps of piles fastened 

 together and extending out from the bank, or parallel to the bank, are 

 especially effective because of the large sediment load of the Missouri. 

 The dikes retard velocity sufficiently to cause deposition behind them, 

 which in turn helps to stabilize the dikes and induce further deposi- 

 tion. In a few months sand fills 10 to 20 feet deep may accumulate, 

 over which flows are retarded sufficiently to deposit silt in which a 

 thick growth of willows soon appears and further stabilizes the deposit. 

 This effects a prograding of the high banks, adding to the area of over- 

 bank flood plain and helping turn the channel toward the opposite side. 

 In some places where the channel impinges against the bluffs, shallow 

 bedrock prevents driving piles, and loose rock dikes are used. The 

 most critical factor is protection of the concave banks against lateral 

 erosion. Bank revetment usually consists of woven willow or lumber 

 mattresses, weighted with stone to sink them into place below the water 

 line, and anchored by piles driven along their landward sides. Above 

 the low-water line the banks are paved with stone (Walsh, 1936) . 



Dredging is necessary to maintain required depth for navigation 

 across many bars during periods of low water, although maintenance 

 dredging is expected to decrease as the channel is more completely 

 stabilized. During the fiscal year 1948 such dredging amounted to 8.5 

 million cubic yards at 48 localities (U. S. Dept. of Army, 1948). 



The regulation works for navigation reduce low-water channel cross 

 sections but this is expected to be offset by increased hydraulic effi- 

 ciency at high stages so that discharge capacity will be at least main- 

 tained, and probably increased. Studies reported by Whipple (1942), 

 following theoretical analyses by Straub (1935), indicated that the 

 improved channel may be expected to scour itself deeper because of 

 increased sediment-carrying capacity. 



The protection of the alluvial lands from bank erosion and channel 

 avulsions is a very important result of the channel stabilization, and 

 construction of a comprehensive system of levees to protect these al- 

 luvial lands from flooding has been started, which would not have been 



