Page 115


Page 115
Colorado River problem
Is Part Of,8
Full text
420 R. A. HILL ON THE COLORADO RIVER PROBLEM This in itself would alter the stage of equilibrium. Certainly the characteristics of a desilted river cannot be based on data acquired when the river was heavily laden with silt. Before the Colorado River is regulated, no one can predict which rate of discharge from a flood-control reservoir will produce the most stable condition. [Fig. 24. SILT VOLUME CURVE - COLORADO RIVER AT YUMA, ARIZ.] Silt which enters the Colorado River above the last reservoir will be intercepted and deposited in this and other reservoirs. The water which is passed down the river will be clear at first, and it will be highly desirable to keep it as clear as possible by reducing scour in the channel. A load of silt, however, will be picked up along the river whenever the rate of discharge is materially increased. As much of this silt will be carried into the irrigation canals, fluctuations in the flow of the Colorado River should be reduced to a minimum. This consideration alone will require that the release from the flood-control reservoirs be held to much less than 80,000 sec-ft., as the irrigation demand will not exceed about 30,000 sec-ft. Estimates of the available water supply and of the future irrigation require ments naturally differ widely, as these are to a considerable extent matters of conjecture. The average annual discharge of the Colorado River at Yuma, as estimated by Herman Stabler, M. Am. Soc. C. E., was 30% greater for the last 20 years than the average for the preceding 20 years. It is believed that the run-off for the years preceding 1902 has been computed too conservatively. Prior to its break into the Imperial Valley, the Colorado River had been lengthening its old channel by deposition of silt at the head of the Gulf of California. This naturally resulted in a continuous raising of the water surface at Yuma, until, as shown in Fig. 4, only about 30,000 sec-ft. were carried at a gauge height of 23.0 ft. Since 1905, largely on account of the change in the river channel below Yuma, the average discharge for the same gauge height has been more than 50,000 sec-ft. The original gauge readings were kept by the Southern Pacific Railroad Company which had little interest in the volume of water in the Colorado River. Consequently, there were not

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