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Page 12
Colorado River problem
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316 THE COLORADO RIVER PROBLEM urated bank will be cut away rapidly. The records indicate that undercutting is likely to proceed more rapidly and to a greater depth the greater the quan-tity of water flowing in the river and, therefore, may get beyond control when the flow is very large. The silt brought down by the river affects the flood problem in three ways: 1.—It accentuates the meandering of the stream and the instability of the channel as just described. 2,—It deposits in the delta and gradually raises the bed of the river, thereby requiring higher levees. This process is slow unless the deposit is concentrated over a small area near a levee as happened with the Volcano Lake Levee from 1909 to 1922. 3.—It may fill the flood channel excessively. Winter floods from the lower tributaries, including the Gila, carry excessive quantities of sediment, much of which deposits in the flood channel. In years when late summer and winter floods are above normal, the flood channel is filled to a point that may cause trouble when the large summer flood starts. This is shown by comparing the gauge heights at a flow of 10,000 sec-ft. (Fig. 4) with the hydrograph (Plate I). [Fig. 4. COLORADO RIVER - RECORD OF GAUGE HEIGHT AT YUMA FOR CERTAIN DISCHARGES] From Laguna Dam to Yuma, the river flows through a broad valley and has all the characteristics of a meandering alluvial stream. At Yuma, it is contracted by solid banks to a width of less than 600 ft. and continues in a channel

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