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silt. Thorough investigation of the evaporation phenomenon is in progress. Precise leveling disclosed that the whole country covered by the lake had subsided a maximum of 7 inches. Gradual filling of the storage space with sediment is not the only silt problem introduced by construction of Hoover Dam and of Davis, Parker, and Imperial Dams downstream. Through the centuries the bed of Colorado River had adjusted itself to transportation of large volumes of silt. In order to moye this weight, a relatively steep slope was required. As the dams were closed, and silt deposited in their reservoirs, the water dis- charged through the dams or power plants became clear. Since transport of clear water'required a flatter slope than transport of a water and silt mixture, the river began to readjust its slope. Establishment of the flatter slope downstream meant lowering of the river bed below the dams. Silt thus removed from the river bed is deposited in the reservoir next downstream. In such depositions the coarser silt is deposited in the upper reaches of the reservoirs, forming deltas similar to those where silt-laden rivers enter the ocean. The result is that the river bed upstream from the reservoir becomes aggraded, a phenomenon which, over a sufficient length of time, may extend a gr~at distance. The initial result of removal of the silt load is, therefore, that the river bed becomes lowered downstream from dams and raised upstream from reservoirs. Lowering of the river bed at an existing gravity diversion may render the diversion no longer possible without a dam. This has occurred at the point of diversion for the Palo Verde Irrigation District. To save this fertile area from destruction due to lack of water, Congress appropriated funds for construction of a temporary diversion dam to maintain the water sur- face elevation at the canal intake in spite of the general lowering of the river bed below Parker Dam. On the other hand, the river bed is increasing its elevation at the drainage outlet from the District. Temporary river rectification has been carried out to provide relief pending a more extensive project. A large storage dam may also cause serious problems downstream by the elimination of flood flows which, in the natural state, served to clean the river bed of accumulated vegetation and silt. This and other causes brought about by construction of Hoover Dam, added to the delta formation upstream from Parker Dam reservoir, created a serious situation along the river between Needles, California, and Topock, Arizona. A true river bed ceased to exist and the water found its way through innumerable sloughs, creating a virtual swamp. To remedy this condition and protect Needles and the Santa Fe Railroad, which parallels the river, the Government in 1948 began a dredging project, which is nearing completion. The water surface at Needles has already, dropped six feet. Large areas of land adjacent to the river, which had become water-logged, are now suited for growing crops. Th~s short outline of problems on the Colorado River shows that con- struction of dams' to control floods and silt does not prevent other problems from arising which require extensive and continuous remedial works and main- tenance. *********** -37-