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6 Table 1.1.--Major dams in the Colorado River basin Total reservoir Drainage River/state capacity area Dam year completed (acre-feet) (sq. mi.) Fontenelle Green/Wyoming 345,360 4,200 1964 Flaming Gorge Green/Utah 3,788,700 15,200 1964 Blue Mesa Gunnison/Colorado 940,700 3,500 1966 Morrow Point Gunnison/Colorado 117,200 3,600 1968 Dillon Blue/Colorado 254,000 335 1963 Crystal Gunnison/Colorado 25,200 4,000 1976 Navajo San Juan/ 1,708,600 3,600 New Mexico 1963 Glen Canyon COlorado/Arizona 27,000,000 108,000 1963 Hoover Colorado/Nevada 29,755,000 167,000 1935 The basin is arid to semi-arid with an average annual rainfall of about 10 inches. The annual precipitation varies from over 40 inches in the higher mountainous areas to less than 3 inches near Hoover Dam. Long cold winters and cool short summers characterize the climate of the mountains in the basin. In the lower areas the winters are mild and short, and the summers are long and warm. The temperature extremes in the basin range from -50oF to 1150F. The average annual runoff is less than 1.5 inches for the entire basin. Most of this runoff is produced in the upper-basin areas. Snow accumulation normally begins in October in the high mountains and in some years continues through May. Vegetation varies from typical desert related plants in the lower basin (cactus, Joshua trees, creosote bush, salt bush, greasewood, desert sage, and mesquite), to sage brush and perennial short and semi-desert grasses in the high plateaus, to typical mountainous vegetation (Juniper, Spruce, and Pinon Pine) in the higher portions of the basins. The vegetation density correlates nearly directly with elevation increase (higher elevation means more dense cover) . Flooding from snowmelt in the basin normally begins in April and May and reaches its maximum in mid to late June or sometimes in early July, depending on the year. Flooding also occurs as the result of