360 fry. SEDIMENTATION IN RESERVOIRS [Ch. 20 



lake with silt. More recent accurate measurements of depositions 

 in Lake Mead give the lake a considerably longer life than Stevens 

 anticipated. However, the fact remains that unless protective meas- 

 ures or other construction is undertaken on the watershed upstream 

 from Lake Mead, the usefulness of that great lake will within a few 

 generations be seriously impaired. The effect would be a progressive 

 return to the run of river conditions of water supply which existed 

 prior to the construction of Hoover Dam, with consequent very large 

 economic losses to irrigation, power production, flood control, and 

 water supply. 



The economic losses that would result from the catastrophe of the 

 filling by silt of such a great reservoir as Lake Mead would be very 

 large. This is probably the most spectacular example to be found 

 within the United States. However, wherever reservoirs are built on 

 streams that carry significant volumes of silt, some degree of economic 

 loss must be anticipated. 



Important water losses may occur as a result of the effects of sedi- 

 ment deposition in reservoirs, particularly in arid and semi-arid re- 

 gions. Evaporation increases because of the relative increase in ex- 

 posed water surface for the same volume of water storage. Trans- 

 piration from vegetation growing on sediment deposits at the heads 

 of reservoirs may consume large quantities of water. For example, 

 it has been estimated that the annual loss by transpiration by vegeta- 

 tion growing on deposits at the head of the reservoir formed by Parker 

 Dam on the lower Colorado River is 400,000 acre-feet (Maddock, 

 1948). 



The loss of use of a good dam site because the reservoir fills with 

 silt is a serious economic problem. Good dam sites are scarce, and 

 the first site selected and used may exhaust the opportunities for 

 economic development on a stream or in an area. If the reservoir 

 is destroyed by silting, the possibilities for building replacement proj- 

 ects are usually limited or non-existent. 



REMEDIES FOR RESERVOIR SEDIMENTATION 



What can be done to cope with the problem is a question that can 

 be answered only for each specific situation. Because the silt origi- 

 nates in the watershed tributary to a reservoir, the most obvious point 

 to prevent silt from being carried into a reservoir is the watershed it- 

 self. A good vegetal cover on a watershed is the best preventive of 

 sedimentation, and it assures long life for a reservoir. For example, 

 the Fontana Reservoir of the Tennessee Valley Authority on the 



