OCR Text |
Show tion and export from the basin, was estimated to be 1,870,000 acre-feet annually. Thus, the actual flow of the Colorado at Lee Ferry is only 14.4 million acre-feet. In comparison to many of the large rivers of the United States, the Colorado's annual flow is small, but because of the aridity of the Southwest, the waters of this river and their wise conservation and use are especially vital to the economy of the region. Before existing projects were built providing par- tial river control, seasonal flow fluctuated greatly. Fed by spring snow melts, the Colorado would be- come a torrent in some areas, overflowing and undercutting its banks and flooding the land for miles. The stream flows of the Colorado today should be considered in two parts: the flows in the river above Hoover Dam, and the completely man- regulated flows below the dam and its impounded Lake Mead. In the section of the river above Hoover Dam, flows of the main stem increase continuously downstream. For the period 1914-45 the average annual flow at the Colorado border was about 6.2 znillion acre-feet, while at Lee Ferry, flow for the same period was 13.8 million acre-feet, and at Hoove*- Dam 14.8 million acre-feet. Below Hoover D>am there is a continuous diminution of flow, due to irrigation, evaporation, and domestic water supply withdrawal, to about 10 million acre- feet avera.ge annual actual flow where the river enters Mexico.2 However, average annual flows do * Department of the Interior. not present a clear picture of stream flow character- istics. For example, just above Lake Mead, the flow varies from a minimum of 700 cubic feet a second to more than 300,000; since the dam was constructed the flow below it has ranged from 1,920 to 25,600 cubic feet a second. The turbulence and sediment-carrying capacity of the Colorado and its tributaries are due in part to stream gradients in the basin. The gradient be- tween the upper basin and the mouth of the river is extremely steep, particularly above the point where the river passes through an escarpment be- tween the high plateau and the lower valley at the Hoover Dam site. It is roughly estimated that 199 million tons of sediment are contributed to Lake Mead each year by the Colorado and its tributaries. The water which is then discharged from Hoover Dam is free of sediment but the amount picked up from the river bed and carried past Yuma averages an estimated 13 million tons yearly. In summary, there are four main characteristics of the basin which give the Colorado River its unusual hydrologic features. (1) Precipitation dis- tribution and character vary greatly over the water- shed; (2) the precipitous gradient of the basin pro- motes rapid discharge; (3) the wide seasonal and annual range in river flow results in an unreliable supply; and (4) the very heavy sediment burden carried and deposited by the river causes a con- stantly changing channel and affects irrigation and other uses. 364 |