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<br />. <br /> <br /><:) <br />00 <br />tv <br />-.J <br /> <br />. <br /> <br />. <br /> <br />DESCRIPTION OF BASIN (Continued) <br /> <br />the Gulf of Mexico, <br />October through April <br />early summer. <br /> <br />The Pacific source predominates generally from <br />and the Gulf source during the late spring and <br /> <br />In the northern part of the basin most precipitation falls in the <br />form of winter snows and spring rains, Summer storms are infrequent but <br />are sometimes of cloudburst intensity in localized areas. In the more <br />arid southern portion the principal rainy season is in the winter months <br />with occasional localized cloudbursts in the summer and fall. <br /> <br />Extremes of temperature in the basin range from 500 F. (10 DC) <br />below zero to 1300 F. (54 DC) above zero. The northern portion of the <br />basin is characterized by short, warm summers and long, cold winters, <br />and many mountain areas are blanketed by deep snow all winter. The <br />southern portion of the basin has long, hot summers, practically con- <br />tinuous sunshine, and almost complete absence of freezing temperatures, <br /> <br />The entire basin is arid except in the extremely high altitudes of <br />the headwaters areas. Rainfall averages as low as 2.5 inches (63,5 mm) <br />in the southern end of the basin while total precipitation in the <br />mountain tops could reach as high as 40 to 60 inches (1020 to 1520 mm) <br />annually. <br /> <br />D. Vegetation <br /> <br />Areas of higher elevation are covered with forests of pine, fir, <br />spruce, and silver-stemmed aspens, broken by small glades and mountain <br />meadows, Pinon and juniper trees, interspersed with scrub oak, mountain <br />mahogany, rabbit brush, bunch grasses, and similar plants grow in the <br />intermediate elevations of the mesa and plateau regions. Large areas in <br />the Upper Basin are dominated by big sagebrush and related vegetation, <br />Many of the streams are bordered by cottonwood, willows, and salt cedar. <br />Scattered cottonwoods and chokecherries grow in the canyons with the <br />cliff rose, the redbud, and blue columbine, A profusion of wildflowers <br />carpets many mountain parks. At lower elevations large areas are almost <br />completely devoid of plant life while other sections are sprinkled with <br />desert shrubs, Joshua trees, other Yucca plants, and saguaro cacti, some <br />of the latter giant plants reaching 40 feet (12 m) in height. Occasion- <br />ally, cottonwoods or desert willows are found along desert streams with <br />mesquite and creosote bush or catclaw and paloverde. Many river flood <br />plains have been overrun with tamarisk or salt cedar to the extent that <br />a large volume of water is being consumed by such vegetation, <br /> <br />E. Hydrolog~ <br /> <br />The <br />(4300 m) <br />National <br />southwest <br /> <br />Colorado River begins where peaks rise more than 14,000 feet <br />high in the northwest portion of Colorado's Rocky Mountain <br />Park, 70 miles (113 km) northwest of Denver. It meanders <br />for 640 miles (1030 km) through the Upper Basin to Lee Ferry, <br /> <br />9 <br />