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~ o en 1;,)1 face waters and not pull cold lfa ter from deeper strata in the reservoir. 680 ( 675 670 665 660 T 655 '" E 6~0 , 6~5 6~O 635 630 0 ) ( Las Vv'llos Wosh Solinit)' Control 100,000 tons yr~1 (t,48X IOBkO )'r-I) ) ( ) Grand Volle)' Salinity Control 195,092 tons .y,-I (1.98 x 108kO yr-I) Evaporation Decrease 120,000 oc-ft {1.4Bx IOBm3)yrl 21?- ,ODD ac fl (2.63 x IOBm3)yr-1 >0--0< 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19 20 YEAR Figure 7. Salinity Model for Lake Mead Projecting Average Reservoir Salt Concentrations for Evaporation Reductions and Various Salinity Control Projects. A second problem that needs to be considered relates to the impacts of warm-water discharges on downstream uses. The Black Canyon area below Hoover Dam supports a popular cold- water trout fishery that could be adversely influenced by warm-water discharges from Hoover Dam. Recent studies, how- ever, have shown that warm-water discharges could benefit reproduction of aquatic insect populations that comprise an important food resource for trout [13J. Aquatic insects re- quire seasonal temperature cycles, like those that existed historically, to complete their life cycle [1'4J. Discharge temperatures from Hoover Dam are now virtually constant at 12-130C throughout the year and appear to be the cause for declines in aquatic insect populations in Black Canyon. Op- eration of Hoover Dam from a surface discharge would re- store seasonal temperature cycles in the river and perhaps enhance production of aquatic insects. This, combined with stocking of warm-water tolerant rainbow trout, could insure that a viable trout fishery was still preserved in Black Canyon. 11, \;