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Description of Basin Table 1.-Comparison of river basin drainage and runoff Area Runoff Runoff River basin (square miles) (million acre-feet per year) (inches per year) Colorado 244,000 15 1.2 Mississippi 1,234,000 440 6.7 Columbia 258,000 180 13.1 Delaware 12,000 14 20.9 The flows at various points in the Basin for the 1941-95 period are given in Appendix A. The records of flow depict wide fluctuations from month to month and considerable variations from year to year. RESERVOIR STORAGE Wet and dry cycles have played a significant role in bringing about the development of the Colorado River reservoir complex (figure 1). Historic records show that the annual flow of the river has varied from less than 6 million to more than 20 million acre-feet per year. The reservoir system allows sufficient storage water to maintain the flows of the river to meet downstream needs during dry periods. In addition to the major reservoirs, numerous smaller reservoirs have been built on many of the tributaries. Major storage began with Lake Mead in 1935 and concluded with the filling of Lake Powell in 1980. The Basin reservoirs now have a combined storage capacity equal to approximately four times the total average annual virgin (undepleted and unregulated) flow of the Colorado River. The flow of the San Juan River is controlled by Navajo Dam, the Green River by Fontenelle and Flaming Gorge Dams, and the Gunnison River by the Wayne N. Aspinall Unit Dams. Glen Canyon Dam is the only major dam on the mainstem of the Colorado River above Lee Ferry, but it controls almost all the flow leaving the Upper Basin. Lake Mead, formed by Hoover Dam, supplies most of the storage and regulation in the Lower Colorado River Basin. Lake Mead provides water for irrigation, municipal and industrial uses, power generation, flood control, recreation, and many other beneficial uses. Lake Mohave, the reservoir formed by Davis Dam, backs water upstream at high stages about 67 miles to the tailrace of Hoover Powerplant. Storage in Lake Mohave is used for some reregulation of releases from Hoover Dam, for meeting treaty requirements with Mexico, and for developing power head for the production of electrical energy at Davis Powerplant. The river flows through a natural channel for about 10 miles below Davis Dam at which point the river enters the broad Mohave Valley 33 miles above the upper end of Lake Havasu. 7