396 brown. EFFECTS OF SOIL CONSERVATION [Ch. 22 



Accelerated sedimentation on flood plains has been traced to all the 

 sources previously described. 



Measures that reduce the frequency of inundation will reduce future 

 depositional and erosional damages, except bank erosion which, with- 

 out bank protection, may possibly be more rapid at bank-full stage 

 than at overflow. Levees and related flood-control works will not re- 

 store soil already impaired or lost. Direct land and channel improve- 

 ments will be required to reclaim lands that already have been dam- 

 aged. Reduction of the sediment supply by watershed treatment or 

 reservoirs, however, will tend to cause channel deepening and, hence, 

 increased discharge capacity, lowered water tables, and reduction in 

 the average acreage damaged. 



Damages to other than agricultural land result from sediment dep- 

 osition on streets, in houses, machinery, sewer lines, and wells, from 

 which it must be removed before the facility can be again used. 

 Much of the loss reported as due to flood damage, however, has, in 

 fact, been due to sediment (Brown, 1945c). Prevention of inundation, 

 of course, will eliminate this damage, but it can be lessened to some 

 extent by any methods that tend to reduce the sediment load of the 

 stream regardless of their effect on flood stage. 



Sedimentation in Reservoirs 



Most impounding reservoirs are constructed for power, irrigation, 

 flood control, water supply, recreation, navigation, or multiple pur- 

 poses. Sedimentation damages a reservoir when it reduces the storage 

 capacity to the point where the reservoir cannot supply the full serv- 

 ices for which it was designed (Brown, 1944; Edgecombe, 1934). 

 Sometimes deposition on beaches, in boating areas, etc., may cause 

 damage even though the capacity loss is negligible. 



Regulating storage is required in connection with most hydroelectric 

 installations in order to maintain a high rate of primary power pro- 

 duction. Any loss of reservoir capacity for such storage, therefore, 

 causes some damage, as it decreases the minimum constant flow that 

 can be maintained through the power turbines. When periods of 

 spillway overflow are followed by periods of low runoff during which 

 the power pool level is drawn down, the direct loss from silting can 

 be measured by the kilowatt-hours of electrical energy that could 

 have been produced by previously wasted water equal in volume to 

 the sediment deposited in the zone of drawdown. Continuity of the 

 rate of power generation makes the difference between primary and 

 secondary power. The greater the fluctuation in power output from 

 hydroelectric plants, the more stand-by sources of power, such as 



