Ch. 20] REMEDIES FOR RESERVOIR SEDIMENTATION 361 



Little Tennessee River is estimated to have a life of nearly 4,000 

 years before it will be filled with silt. This long life is due primarily 

 to the watershed having a cover of trees and other vegetation. On 

 the other hand, a small reservoir above a diversion dam on the Ocoee 

 River located below a denuded watershed, because it traps the sedi- 

 ment eroded from the denuded land (Tennessee Valley Authority, 

 1949b) , is estimated to have a useful life of not more than half a cen- 

 tury, at which time certain operating advantages derived from the 

 reservoir will be lost, although the main purpose of the project, to 

 divert water, will not be affected. 



The success of watershed protective measures depends both on the 

 climate and on the management of the land in the watershed. If 

 either of these is not favorable, reservoir silting is certain to be ag- 

 gravated. Improvement of the watershed cover is extremely difficult 

 and may be practically impossible in those climatic regions where 

 precipitation occurs only occasionally and then in very intense storms 

 (Peterson, 1948). In such watersheds, sod-forming grasses are un- 

 common in nature, and evidences of geologic erosion are on every 

 side. 



Once sediment has been deposited in a reservoir, disposal is ex- 

 tremely difficult and practically impossible. In isolated cases, density 

 currents have carried some silt through a reservoir (Bell, 1942; U. S. 

 Department of Interior, 1949). However, the effect of density cur- 

 rent flow on the amount of sediment deposited within the reservoir is 

 necessarily small. On some projects, it is possible to carry silt through 

 a dam by means of sluices through the structure. Ordinarily, how- 

 ever, this is not feasible, because to reduce materially the sediment 

 content of a reservoir would require a long period of sluicing during 

 which the reservoir would be out of use. Dredging in a few special 

 cases may be resorted to, but this method is not generally applicable. 



Perhaps engineers should pay more attention in the original design 

 to provide for eventual sluicing of silt through a dam. At the time 

 when the amount of sediment deposited in a reservoir becomes suffi- 

 cient to interfere seriously with the purpose or purposes of that reser- 

 voir, it might be practical to abandon the uses of the reservoir and 

 open sluices and carry away through the dam the silt that has been 

 deposited. How long this would take would depend on the amount of 

 silt, stream flow, or other local conditions. But it would probably be 

 cheaper to carry out such de-silting operations over a considerable 

 period of time, even for rather large reservoirs, than to construct en- 

 tirely new reservoirs. Looking considerably ahead, if conditions 

 warrant, when many reservoirs now constructed become silted to the 



