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<br />. <br /> <br />. <br /> <br />Lv <br />CO <br />('.0._ <br />N <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />the southeastern portion of the basin and flows westward to meet the <br /> <br />Colorado River about 75 miles west of Bluff, Utah. The other major tri- <br /> <br />butary to the Colorado River is the Gunnison River, which drains the <br /> <br />northern slope of the San Juan Range and part of the western slope of the <br /> <br />Rocky Mountains. <br /> <br />A broad range of climate and streamflow conditions exist within the <br /> <br />Upper Colorado River region. The basic inflow, or input of water to any <br /> <br />hydrologic system, originates in the form of precipitation. Strea mflow <br /> <br />is defined as that portion of the precipitation which appears in streams <br /> <br />and rivers as the net or residual flow collected from all or a portion of <br /> <br />the watershed. Annual precipitation varies from over 50 inches in the <br /> <br />high-elevation headwaters to less than 6 inches in desert areas of the <br /> <br />southwestern portion of the region. As indicated by Figures lll-l and IIl-2, <br /> <br />most of the streamflow is provided by snowmelt from the mountainous <br /> <br />areas, which produces high rates of runoff during the snowmelt period <br /> <br />each year. Because almost 50 percent of the Upper Colorado River <br /> <br />Basin receives less than l2 inches average annual precipitation, most of <br /> <br />the interior tributary streams are ephemeral, with water flowing in them <br /> <br />only after infrequent storms. <br /> <br />Historic unit discharge rates on the main stem, as shown by the <br /> <br />tabulation in Table IIl-l, decrease rapidly as the tributary streams flow <br /> <br />from their headwaters at high altitudes into the less-humid areas and <br /> <br />finally into desert areas. <br /> <br />13 <br />