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Show tained. As these break down, quantities of debris are loosed into the streams. During the early pe- riod, no detention structures were required and the tailings were sluiced directly into the stream. Most of the waste material from strip mining, placer min- ing, and quarrying washes into the streams today. This material is in transit toward the lower reaches.62 Sediment production following dam construc- tion.-With the construction of Hoover Dam and the formation of Lake Mead, the sediments could no longer be carried downstream, but were de- posited in the lake. Clear water was discharged into the river below the dam. The scouring power of clear water in a river bed with a slope of 1.5 to 2 feet per mile is great. Immense quantities of river bed material were picked up and transported fur- ther downstream. Sediment amounting to ap- proximately 36 million tons per year has been eroded from the river bed below the dam. Prior to construction of Parker and Imperial Dams farther downstream, the finest of this eroded material was carried to the lower reaches of the river, the coarser material being deposited in quiet places along the stream. These deposits caused a rise in river stages in such places as Needles, where a part of the city was flooded. They plugged the drainage outlet for the Palo Verde Irrigation Dis- trict, causing swamping in the lower end of the district. With the closure of Parker Dam in 1938, and the resulting release of clear water, the river bed was freshly attacked at another point. Materials in amounts up to 41 million tons a year were eroded. This erosion extended downstream beyond the in- take of the Palo Verde Irrigation District, lowering the river bed at the intake so that the district could not divert its full requirement of water. Until early in 1946, the major portion of the ma- terial eroded below Parker Dam had been trapped by Imperial Dam, 150 miles farther downstream. The water leaving Imperial Dam with a reduced sediment load has a renewed scouring power, at- tacking the sediment on the river bed. Large quantities were removed below Laguna Dam, and carried downstream past the International Boun- dary. By 1946, the area above Imperial Dam was filled with sediment, and almost the entire load eroded below Parker Dam was carried past Imperial Dam. With construction of Davis Dam 65 miles below U. S. Forest Service. Hoover Dam, no further retrogression of the river bed will take place between the two dams. Con- tinued changes will take place, however, in the reach between Davis Dam and Topock at the head of Havasu Lake. A free-flowing channel from Needles to Topock should correct some of the difficulty. To restore the Palo Verde Irrigation District di- version elevation, a temporary rock weir was built across the river and a new headworks constructed. The Indian Service, on behalf of the Colorado River Indian Reservation, desires that the weir be re- moved to permit a lower water surface elevation and resultant improved drainage conditions at the lower end of the reservation. A cooperative study by the Indian Service, Bureau of Reclamation, and the irrigation district is attempting to provide a permanent solution to the problem. A 12-mile channel through the Cibola Valley is proposed as a solution. Experience in corrective measures.-What is pos- sible through corrective land management is shown by the Mesa Verde National Park, which once was part of the overgrazed public domain but now has recovered. (See problem C-2.) Several of the States have laws which require that no waste material may be dumped into the streams. Many of these laws have fallen into dis- use. Were they enforced, at least some of the deposition of sediment material into the streams would quickly cease. A remedial program is currently under way on the public lands under Federal administration. On the national forests, a program of range re- seeding and rehabilitation is under way. This calls for some stock reductions, permanently in some places, temporarily in others. This program, how- ever, has started rather late because erosion con- ditions have already reached a critical stage in many areas. (See problem C-2.) State lands in the basin are about in the same condition as the public range lands. Intermingled private lands are generally in the same condition as the Federal lands. Conclusions (1) To reduce sedimentation a large program of land rehabilitation is needed. All of the Federal agencies recognize the situation and want to correct it. However, the unavailability of funds for con- servation purposes is retarding a constructive program. 433 |
