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<br />The Colorado River Basin is an arid or semiarid basin. Compared to <br />others, such as the Columbia Basin which drains approximately the same area, <br />it carries a smaller flow, as shown in the following table. While the <br />Colorado River is one of the major drainage basins in the continental United <br />States, its runoff is about equal to that of the Delaware River which drains a <br />much smaller area. <br /> <br />I-'" <br />-..1 <br />~ <br />C",t) <br /> <br />Table II-i. Comparison of river basin drainage and runoff. <br /> Area Runoff Runoff per <br /> (1,000 square (million acre- unit area <br />River basin miles) feet per year) (inChes/~ears) <br />Colorado 244 15 1.1 <br />~Mississippi 1,234 440 6.7 <br />Columbia 258 180 13.1 <br />Delaware 12 14 20.9 <br /> <br />The flow at various points in the Colorado River Basin for the 1941-83 <br />period is given in Tables 1 through 20 at the end of this report. The records <br />of flow depict wide fluctuations from month to month and considerable <br />variations from year to year. The storage reservoirs presently reduce some of <br />the fluctuation in the reaches below the major dams. <br /> <br />C. Reservoir Storage <br /> <br />Wet and dry cycles have played a significant role in bringing about the <br />development of the Colorado River Reservoir complex. In the past, the annual <br />flow of the river has varied from less than 6 million to over 20 million <br />acre-feet per year. The reservoir system allows sufficient storage water to <br />maintain the flows of the river to meet downstream needs during dry periods. <br /> <br />The construction and filling of the main stem reservoirs of the Colorado <br />River Basin have brought about significant changes in the flow patterns of the <br />river. In addition to the major reservoirs, numerous smaller reservoirs have <br />been built on many of the tributaries. Since major storage began with Lake <br />Mead in 1935 and concluded with the filling of Lake Powell in 1980, the <br />Colorado River Basin reservoirs now have a combined storage capacity equal to <br />approximately four times the total average annual virgin (undepleted) flow of <br />the entire Colorado River. <br /> <br />The flow of the San Juan River is controlled by the Navajo Dam, the Green <br />River by Fontenelle and Flaming Gorge Dams, and the Gunnison River by the <br />Wayne N. Aspinall Unit Dams. Glen Canyon Dam is the only major dam on the <br />main stem of the Colorado above Lee Ferry, but it will permit control of <br />almost all flows leaving the Upper Basin. <br /> <br />Lake Mead, formed by Hoover Dam, supplies most of the storage and <br />regulation in the Lower Colorado River Basin, providing for irrigation, <br />municipal and industrial uses, power generation, flood control, recreation, <br />and many other beneficial uses. <br /> <br />Lake Mohave, the reservoir formed by Davis Dam, backs water at high <br />stages about 67 miles upstream to the tailrace of Hoover Powerplant. Storage <br /> <br />II-2 <br />