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I ,) . . have been nearly empty in 1941; ,[ould have since partiilly filled; but again would have been dra\lll down to approach the dead stor~ge level by 1954. The experience of the past provides assurance that the active capacity of the reservoire, emptied during prolonged drouths I!ould agdn refill as normal and ~etter years return. In the 191~-1929 period, for instance, the river fl011 at Lee Ferry Itould have been sufficient to satisfy the compaot alloca- tion of the Upper Bssin, the required delivery to the Lower Basin, and pro- vi~e 36 lnillion acre-feet of reserve storage--enough to replenish the active capacity of all storage project reservoirs It times. Over the 25-year general drouth period from 1930 ~~rough 1954, project reservoirs would have smoothed out the seasonal and annual fluctua- tions of streamflow and Itould have made available an additional one million acre-feet per annum of 'tater held over from a ~~tter period. In possibly 10 years distributed throughout the 25-year period serious water shortages \!ould have still e):isted at points o~ use in the Upper Basin. Under extreme condi tions there might also have been some curtaiJm(mt in both Upper Basin and Lower Basin use to meet conditions of the Mexican Treaty. Nevertheless, If.lth the full storage capacity planned for the Colorado River Storage proj- ect, the Upper Basin can plan to eventually consume its 7.5 million acre- feet annually,dth shortages that will not exceed those that ~re tolerable in the design of projects to utilize water for irrigation and other pur- poses. In a river bssin where water is so scarce and so valuable evaporation must be held to a m~n1mum cons~stent with economy ~n project deaign and operation. This has been the objective of the Bureau of Recla- mation in the formulation of the storage project plan, evaporation losses being reckoned as charges a~ainst apportioned consumptive USGS. B,y using methods of calculation accepted by engineers, we have estimated evaporation rates under varying conditions at locations throughout the Upper Basin and thus have established a basis for comparing the relative merits of each reservoir site ,,1. th respect to evaporation. In a ccmplete draltdo\lll and refill cycle the nine reservoirs of the ultimate plan ,[ould lose through evaporation an average of approximately 830,000 acre-feet annually WhiCh approximates the nomal evaporation loss at La](e Head. 1men the reservoir content is 101.[ in periods of drouth the evaporation loss I.[ould be less than average. In the selection of reservoir sites sedimentation is also an important factor. Glen Canyon, the largest storage site in the Upper Basin, lies just above Lee Ferry in the region that contributes the bulk of the river's sediment load in the Upper Basin. Plens for the Glen Canyon unit includes a reservation of about 15 million acre-feet of storage capacity for sediment accumulation. This capacity, in conjunction with sediment capacity planned for other ups,tream reservoirs, is based on catchine a sedi- ment load averaging 100,000 acre-feet a .~ar for 200 years. Even If.lth its sediment handicap, Clen Canyon is one of the most attractive storage Dnd pOlter sites in the Upper Basin. Construction at this site Ilould also pro- vide a long addition to the life of Lake Mead at Hoover Dam and Ifould probably make feasible the development of same intervening hydroelectric 5 1713