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, !: I , rl I I I " .--; "" ~ 0- cluded beca~~ t41~t are operated at relatively constant levels. In addition to surface storage, water is absorbed in the areas adjacent to the reservoirs. At Lake Powell, rhis absorption or bank storage was estimated to be about 3,500,000 acre-feet as of the end of water year 1967, or 42 percent of the gross surface storage. As can be seen in Plate 5 the low runoff for 1967 has delayed initial filling of major basin reservoirs to the combined rared head capacity. Plate 6 shows the changes in contents of the major mainstream reservoirs during the 1967 water year. Lake Mead Sedimentation All plates and tables in this report showing storage at Lake Mead are based upon area-capac- ity rabIes prepared from a hydrographic survey conducted by the Bureau of Reclamation during 1963 and 1964. The last previous sedimentation survey was made in 1948--49. \\Then Hoover Dam was completed in 1935, the reservoir had an active storage capacity of , 28,030,000 acre-feet at top of spillway gates, ele- vation 1221.4 feer above sea level. Sedimenr en- croachment reduced the activc capacity at gate top to 27,209,000 acre-feet in 1949 and to 26,- 159,000 acre-feet in 1964. Findings from the 1948--49 and 1963-64 surveys are compared in Table 5. I 1 , I I Table 5 LAKE MEAD SEDIMENTATION /935 1935-1949 1949-1964 Sediment Accumulation, ac.ft. TotaL _ _ _ ____ ___ ______ I,H~,OOO 1,292,000 Annual average_________ 104,000 80,750 1935 1949 1964 Water storage space avail- able, ac. ft. Active_________ ________ 28,030,000 27,209,000 26,159,000 Dead_________. ________ 3,223,000 2,620,000 2,378,000 The average annual rate of sediment accumu- lation in Lake Mead was 22 pcrcent less during the 16 years 1949 through 1964 than during the 14 years from 1935 through 1948, The decrease during the later period is attributed by the Bu- reau principally to the lower average runoff and to small extent to the closing of the gates in Glen Canyon Dam in 1963, Future survevs should make it possible to segregate more readiiy the effects of Glen Canyon Dam and other up- stream structures in reducing sediment inflow to Lake Mead. Uses and Losses Depletions in rhe Upper Basin are not all meas- ured. The Bureau of Reclamation computes irri- gation depletions by applying a unit rate to an estimated acreage. The unit rate is derived for each year by applying to the estimated long-time average a factor varying with the annual runoff, indicating uses greater rhan average in years of high runoff and less than average in years of low runoff. This type of adjustmcnt is questionable for application to present development because of the increasing amount of storage regulation available to supplement low runoff. Including transmountain diversions which are measured, and evaporation from reservoirs, total depletion in 1966-67 is estimated to be 2,480,000 acre-feet, an increase of 450,000 acre-feet from the esti- mated 1965-66 depletion, Diversions minus measured returns to the river by Lower Basin mainstream users in water year 1966-67 were 5,873,000 acre-feet, an increase of 127,000 acre-feet from 1965-66. Quantities for the past five years ate shown in Table 6. Table 6 DIVERSIONS MINUS MEASURED RETURNS LOWER COlORADO RIVER (Thousands of Acre-feet) H~Q.tu Yt/3r Cahfornia Palo Verde J rrig. Dist. Metropolitan Water Dist.h____________ Yuma Project Reserv. Div.. _ ____________ Imperial Irrig. Dist.__ Coachella Valley Co. Water DisL_______ 1963 1964 1965 1966 1967 362 403 373 384 365 1,065 1,092 1,180 1,121 1,182 45 48 46 ~8 51 3,053 2,859 2,756 2,778 2,860 537 505 526 484 453 5,062 ~,907 4,881 4,815 4,911 182 189 178 186 200 591 642 616 555 566 171 176 182 162 I71 944 1,007 976 903 937 l'otaL __ __ _ _ _. __ Ar1.tona Colorado R. lndian Reservation__ _ _ _ _ __ Gila Project. _ n __ ____ Yuma Proj. Valley Div.______________ TotaL__________ N nJada Pumping (rom Lake ~ead_____________ 26 27 23 25 27 Gcand TotaL_ 6,032 5,941 5,880 5,743 5,875 31