Ch. 22] SEDIMENTATION IN RESERVOIRS 397 



steam plants, must be operated. To the extent that this is necessi- 

 tated by reduced storage capacity, the silting damage may be meas- 

 ured by the added cost of operating stand-by plants. Damages may 

 thus be said to start with the beginning of storage and sedimentation. 



Sedimentation damage to irrigation reservoirs is likewise measured 

 primarily in terms of the value of water wasted over the spillway 

 that could have been retained and used. Some of the largest reser- 

 voirs in the Southwest (Elephant Butte, New Mexico; San Carlos, 

 Arizona; and Roosevelt, Arizona) were built with sufficient capacity 

 to impound two or more times the average annual stream flow. Water 

 wastage over the spillway may be expected, therefore, only in years 

 of exceptional flood discharge; and water deficiency attributable to 

 this loss during exceptional droughts may have an average recurrence 

 interval as long as 25 years or more. With cumulative storage loss 

 from sedimentation, however, the recurrence interval of water de- 

 ficiency progressively decreases. Hence crop losses will become more 

 frequent. Theoretically, and often practically, damage begins with 

 the initiation of storage. Furthermore, sedimentation in irrigation 

 reservoirs tends to increase the evaporation loss by increasing the 

 surface area of water exposed for any given volume of water in stor- 

 age. Sediment deposits in the upper end of reservoirs generally be- 

 come covered by water-consuming vegetation, and the resulting heavy 

 evapo-transpiration, especially in the more arid states, may represent 

 a critical loss of available water. (For reports on recent surveys of 

 irrigation reservoirs, see Seavy, 1948a, b, 1949.) 



If a flood-control reservoir is designed to prevent overflow below it 

 from a flood flow of 100-year average recurrence interval, a calculable 

 damage results if its detention capacity is reduced by sedimentation 

 to the point where it can control fully only a 75-year flood flow. In 

 this case also sedimentation damage begins as soon as the reservoir is 

 completed. Flood control is often provided, however, by gate-con- 

 trolled detention storage above the pool level maintained for power 

 production, water supply, or irrigation (U. S. War Department, 1943) . 

 In such multiple-purpose reservoirs, loss of flood-water-detention ca- 

 pacity ordinarily occurs slowly because of the relatively infrequent 

 and short periods of impoundment. Flood control may not be seri- 

 ously affected until the storage capacity at lower levels is greatly 

 depleted. Some reservoirs, on the other hand, are constructed solely 

 for flood control, with large outlets at the base of the dam. The out- 

 lets, gated or ungated, are generally designed so that flood-water de- 

 tention will not begin until the flow downstream is near bank-full 

 stage. Much of the water passes through such reservoirs at normal 



