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. DESCRIPTION OF BASI~ .~ l\; most other parts of the United States. ~ost of the moisture for precip- itation on the Upper Basin is derived from the Pacific Ocean and the Gulf of Mexico. The Pacific source predominates generally from October through April and the Gulf source during the late spring and early summer. .... C> In the northern part of the basin most precipitation falls in the form of winter snows and spring rains. Summer storms 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 months with occasional localized cloudbursts in the summer and fall. Extremes ot temperature in the basin range from 500 F. below zero to 1300 F. above zero. The northern portion of the basin is character- ized by short, warm summers and long, cold winters. and many mountain areas are blanketed by deep snow all winter. The southern portion of the basin has long, hot summers, practically continuous sunshine. and almost complete 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 inches in the southern end of the basin while total precipitation in the mountain tops could reach as high as 40 to 60 inches annually. D. Ve~etation Areas of higher elevation are covered with forests of pine, fir, spruce. and silver-stemmed aspens. broken by small glades and mountain 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. Large areas in the Upper Basin are dominated by big sagebrush and related vegetation. Xany of the streams are bordered by cottonwood. willows, and salt cedar. 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 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. Xany river flood plains have been overrun with tamarisk or salt cedar to the extent that a large volume of water is being cons~~ed by such vegetation. E. Hvdrologv . The Colorado River begins where peaks rise more than 14.000 feet high in the northwest portion of Colorado's Rocky ~ountain National Park, iO miles northwest of Denver. It meanders southwest for 640 miles 7