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6 Chapter 2 -Desoriptlon of Basin HYDROLOGY f\,) c..,) -.:i ton The Colorado River begins where peaks rise more than 14,000 feet in the northwest portion of Rocky Mountain National Park in Colorado, 70 miles northwest of Denver, It meanders southwest for 640 miles through the Upper Basin to Lee Ferry, the dividing point for the upper and lower portions of the Basin, The Green River, the major tributary of the Colorado River, rises in western Wyoming and discharges into the river in southeastern Utah-730 river miles south of its origin and 220 miles above Lee Ferry, The Green River drains 70 percent more area than the Colorado River above their junction, but produces only about three-fourths as much water, The Gunnison and San Juan Rivers are the other principal tributaries of the Colorado River in the Upper Basin, The Colorado River Basin has a total area of approximately 244,000 square miles, carrying an average annual virgin flow of about 15 maf at Lee Ferry, Of this flow, more than 5 mafper year are exported to the Arkansas and Missouri River Basins, the Great Basin, southern California, and the Rio Grande Basin, The Colorado River Basin is a relatively arid basin, Compared to others, such as the Columbia River Basin, which drains approximately the same area, it carries a much smaller flow, Table 1 shows that while the Colorado River Basin is one of the major drainage basins in the continental United States, its runoffis about equal to that of the Delaware River which drains a much smaller area, River basin Table l,-compa~son 01 ~ver basin drainage and runoff Area (1,000 square miles) Runoff (Inches per year) 1,2 6,7 13,1 20,9 Runoff (mal per year) Colorado Mississippi Columbia Delaware 244 1,234 25S 12 15 440 180 14 The flows at various points in the Colorado River Basin for the 1941-91 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 (refer to figure 1 ), Historic records show that the annual flow of the river haa varied from less than 6 million to more than 20 maf 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 Colorado River Basin reservoirs now have a combined storage capacity equal to approximately four times the total average annual virgin (undepleted) flow of the Colorado River, The flow of the San Juan River is controlled by the 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 main stem 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