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Show Viewed in terms of maintaining an American civilization in the Southwest, the sedimentation of reservoirs is serious. Judged in the light of present knowledge of potential hydroelectric power for in- dustrial and agricultural life and of available do- mestic and industrial water, the sediments of the basin must be regarded as a menace. Present plans call for the allocation of some 20 million feet of reservoir storage to sediment deten- tion. This is 43 percent of all the storage space behind 10 large dams proposed for construction in the river system above Lake Mead. Yet this stor- age capacity is sufficient to insure full capacity operation of the facilities for only two centuries at present sedimentation rates. Although this ap- pears long1 to an individual, it is a short time in the history of a people. The cost of storing this sediment behind dams will be great. At the Glen Canyon Dam, for example, preliminary figures indicate that the 15 million acre-feet to be stored there will require an outlay in excess of 140 million dollars. This will be at the rate of $9.40 per acre-foot of sediment re- tained. Vet, if full development of the basin is to be achieved, provision for sediment storage must be made.4* A survey by the Department of Agriculture, the Navy Department, and the Bureau of Reclamation of Lake M^ead in 1949 revealed that approximately 4.4 percent of the original reservoir capacity of this lake had been lost by sediment during the first 14 years of its life. But long before the lake is filled, unless vigorous steps are taken to halt the sediment, it will interfere with the reservoir operation. That not all tlie sediment problem is in the main Colorado is shown by the fact that San Carlos Reser- voir on the Gila River had lost 4.54 percent of its capacity in 18 years, an annual depletion rate of 0.25 percent. Roosevelt Reservoir on the Salt River lost nearly 9 percent of its capacity in 37 years with an annual depletion rate of 0.27 percent as indicated in the following summary: Total sediment deposited acre-feet 0 Year survey: 1905__________________ 1914___________________ 27, 000 1916___________________ 62,000 1935___________________ 108, 800 1946___________________ 140,620 Increase acre-feet 0 27, 000 35, 000 46, 800 31,820 Annual sediment increments in Roosevelt Reser- voir range from 900 to 17,500 acre-feet with an average sedimentation rate of 3,780 acre-feet. Sediment accumulation fluctuates with runoff. The heavy deposition during 1915-16 was during a period of high runoff, the river discharge into the reservoir being almost equivalent to 6 years of nor- mal flow. During this time, the channels were swept nearly clean of all debris. During the dry periods of 1917 to 1919 and of 1925 to 1935, the runoff was exceedingly low and very little sediment entered the reservoir.49 The erosion rate in the basin accelerated during the period from 1870 to 1890. It was during this period that the effects of extensive overgrazing be- gan to be felt. At the same time, large amounts of timber were cut to provide fuel and railroad ties. Range and forest fires were common throughout the basin. The wholesale removal of the vegetal cover produced surface conditions which increased the surface runoff, and occasioned erosion. The damage in the stream caused by increased erosion was not realized particularly until structures were built in the channels. As dams and reservoirs increased in size, their efficiency to trap sediments increased. Today dams are proposed to prevent the sedimentation of other reservoirs. Sources of sediment.-No accurate information exists as to the main sources of sediment. Except- ing for the San Juan, little information is available on the sediment contribution of any individual tributary or area above the mouth of the Little Colo- rado. Yet gaging station data show that some 60 to 70 percent of the sediment carried by the river originates above Lees Ferry.50 A number of theories as to the origin of the sedi- ment load have been advanced including diastro- phic changes, climate, and man. (See Problem G-2.) Some important reasons for sediment in the streams can be found in the storm precipitation of the basin. How storms can affect the erosive process is revealed by the following analysis of sum- mer storms in the Gila Basin: 51 M Bureau of Reclamation. 49 Survey data from Salt River Valley Water Users Asso- ciation, Soil Conservation Service, and Bureau of Reclamation. 60 Proceedings, Federal Inter-Agency Sedimentation Conference, Salt Lake City, 1950. Statement of H. V. Peterson, U. S. Geological Survey. M National Resources Planning Board-Upper Gila River Basin Report, 1941. 430 |