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, I, . ~ "1 ,. . .~:.; ;j .. i.1 HYDROGRAPHY fornia is 64 per cent of the amount specified in contracts with the United States nearly 20 years ago of 5,362,000 acre-feet annually, which is considered as permanently availab]e when needed, by the terms of the Colorado River Compact. Silt ill Lake Mead abuve Hoove.. Dam The first sedimentation survey of Lake Mead was made between March 1948 and March 1949 by the U. S. Bureau of Reclamation in cooperation with the U. S. Navy and the U. S. Geologica] Survey, to determine the accumulation of silt in the reservoir since the gates at the dam were first closed on February 1. 1935. Preliminary provisional data indicate a total of about 1,400,000 acre-feet of silt deposits in this period of 14 years, or an average of approximately 100,000 acre-feet per year. Runoff for the period averaged 97 per cent of the mean of record at the Grand Canyon gaging station, but only 90 per cent of the 53-year hbtorical long-time mean. as extended by estimates frum other records and adjusted for present depletion. The long-time mean silt inflow may therefore be on the order of about 11 0.000 acre-feet annually (or 0.3 per cent of the original capacity of the reservoir), prior to the construction of other planned reservoirs upstream, sucb as the Bridge Canyon project. The "official" pre-Boulder dam silt estimate of the Sibert Committee was 137.000 acre-feet annually, so the present tenta- tive determination is only 80 per cent of the anticipated silt encroachment. . Use of the revised capacity tabre for operation of Lake Mead and Hoover Dam began October 1, 1949, when the active storage at the water surface elevation of 1,186.65 feet (or 35 feet below the spillway elevation) was reduced from 22,798,000 acre-feet to 22.082.000 acre-feet. The silt accumulation in this major portion of the active or usable reservoir capacity was then about 716,000 acre-feet. At elevation 1,045 feet, the sill of the upper outlet in the intake towers. the usable storage capacity is reduced from 7,88:3,000 acre-feet to 7,602.000 acre-feet, a difference of 281,000 acre-feet for the. approximate sediment volume. Below elevation 895 feet, the sill of the lowest outlet gates, is dead or unusable capacity, formerly 3,207.000 acre-feet but now determined as 2,620.000 acre- feet. or silt volume of 587,000 acre-feet. Thus over 40 per cent of the silt storage to date is located in the bottom dead capacity, reducing this by 18 per cent of its original capacity. Of course the reservoir functions just as well whether this dead capacity is filled with water or silt, except that its gradual