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~ N .,!::. -J PART II. DESCRIPTION OF BASIN A. Climate Extremes of temperature in the Colorado River Basin range from -50 to 130. F. The northern portion of the basin is characterized by short) warm summers and long, cold winters; and many mountain areas are blanketed by deep snow all winter. Much of the Intermountain area con- sists of high basins or valleys with cold winters and hot dry summers. The southern desert portion of the basin has long, hot summers, prac- tically continuous sunshine, and almost complete absence of freezing temperatures. Rainfall averages 2.5 inches per year in the southern end of the basin, while total precipitation in the mountains reaches 40 to 60 inches annually. B. Kydrology The Colorado River begins where peaks rise more than 14,000 feet 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, 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 13 to 15 million acre-feet at Lee Ferry. Of this flow, more than 5 million acre-feet per year are exported to the Arkansas and Missouri River Basins, the Great Basin, southern California, and the Rio Grande River Basin. The Colorado River Basin is an arid or semiarid basin. Compared to others, such as the Columbia Basin, which drains approximately the same area, it carries a smaller flow, as shown in the following table. While the Colorado River is one of the major drainage basins in the continental United States, its runoff is about equal to that of the Delaware, which drains a much smaller area. Comparison of river . River basin Colorado Mississippi Columbia Delaware Area (1,000 square mi les) 244 1,234 258 12 basin drainage Runoff (million acre- reet per year) 15 440 180 14 Runoff per unit area (inChes/years) 1.15 6.7 13.1 20.9 8