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. . , . l-" o .::.. Ul DESCRIPTION OF BASIN (continued) In the northern part of the basin most precipitation falls in the form. of winter snows and spring rains. Summer stonns are infrequent but are sometimes of cloudburst intensity in localized areas. In the more. arid southern portion the principal rainy season is in the winter nlontlls with occasional localized cloudbursts in the sumer and fall. Extremes of temperature in the basin range from -50. F (-46. C) to 130. F (54. C). 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. The southern portion of tlle basin h&s long, hot summers, practically continuous sunshine, and almost cOr.J.plete absence of freezing temperatures. The entire basin is arid except in the extremely high altitudes of the headwaters areas. Rainfall averages as low as 2. 5 i~ches (64 lnrd) in the southern end of the basin while total precipitation in the Iilountain tops could reach as high as 40 to 60 inches (1000 to 1500 mul) annually. D. Vegetation Areas of higher elevation are covered with forests of pine, fir, spruce, and silver-stemmed aspens, bro~en by small glades and mountaLl meadows. Pinon and juniper trees, interspersed with scrub oak, mountain mahogany, rabbit brush, bunch grasses, and similar plants grow in the intermediate elevations of the mesa and plateau regions. Lorge areas in the Upper llasin are dominated by big sagebrush related vegetation. Hany of the 6treffi~S are bordered by cottonwood, willows, and salt c~dar. Scattered cottonwoods and chokecherries grow in the canyons with the cliff rose, the redbud, and blue columbine. A profusion of wildflowers carpets many mountain parks. At lower elevations large areas are almost completely devoid of plant life while other section~ are sprinkled with desert shrubs, Joshua trees, other Yucca plants, and saguaro cacti, some of the latter giant plants reaching 40 feet (12 m) in height. Occasionally, cottonwoods or desert willows are fo'und along desert streams with mesquite and creosote bush or catclaw and paloverde. Many river flood plains have been overrun with tamarisk or salt cedar to the extent that a large volume of water is being consumed by such vegetation. E. Hydrology The Colorado River begins where peaks rise more than 14,000 feet (4jOO m) high in the northwest portion of Colorado's Rocky Mountain National Park, 7U miles (113 km) northwest of Denver. It meanders southwest tor 640 miles (103U km) through the Upper Basin to Lee Ferry. The Green River, its major tributary, rises 1n western Wyoming and discharges into the Colorado River in southeastern Utah - 730 river I~il~s (1170 km) south of its origin and 220 miles (350 km) 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 9