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(~ w ,..... "'>.J DESCRIPTION OF BASIN carpets many mountain parks. At lower elevations large areas are almost completely devoid of plant life while other sections are sprinkled with desert shrubs, Joshua trees, other Yucca plants, and saguaro cacti, some of the latter giant plants reaching 40 feet in height. Occasionally, cottonwoods or desert willows are found along desert streams with mes- quite and creosote bush or catclaw and paloverde. In recent years many river channels have been overrun with tamarix or salt cedar to the ex- tent that a large volume of water is being consumed by such vegetation. Measures are being taken to curb the growth of phreatophytes to conserve water. E. ILydroloiW The Colorado River begins where peaks rise more than 14,000 feet high in the northwest portion of Colorado's Rocky Mountain National Park, 70 miles northwest of Denver. It meanders southwest for 640 miles through the Upper Basin to Lee Ferry. The Green River, its major tributary, rises in western wyoming and discharges into the Colorado 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 the San Juan are the other principal tributaries of the upper Colorado River. The flows of the San Juan River are now controlled by the Navajo Dam, the Green River by Fontenelle and Flaming Gorge Dams, and when the Cure- canti Unit Dams are completed, the flows of the Gunnison will be largely controlled. Glen Canyon Dam is the only major dam on the main stem of the Colorado above Lee Ferry, but it will permit control of almost all flows leaving the Upper Basin. The flow at various points in streams in the Colorado River Basin for the 1941-1964 period is given in Tables 1 through 17. The records of flow depict the characteristic wide fluctuations from month to month and the considerable variation from year to year. The recently con- structed storage reservoirs will now level out some of these fluctuations. The natural drainage area of the lower Colorado River below Lee Ferry and above Imperial Dam is about 75,100 square miles. This sec- tion of the river is now largely controlled by a series of storage and diversion dams starting with Hoover Dam and ending at Imperial Dam. At the present time there is no significant storage on the main river or on the tributaries between Glen Canyon Dam and Lake Mead. The intervening tributary inflow is erratic but amounts to almost enough to offset the evaporation from Lake Mead. 8