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1. The Upper Colorado River Basin <br />The Upper Colorado River basin lies in west-central Colorado, with the headwaters originating at the <br />Continental Divide in Rocky Mountain National Park. The Upper Colorado River flows in a westerly <br />direction through forested mountains and irrigated valleys before it leaves the state in Mesa County <br />downstream of the City of Grand Junction. The basin encompasses all or a large majority of Grand, <br />Summit, Eagle, Garfield and Pitkin counties, and portions of Mesa, Routt, and Gunnison counties in <br />Colorado. Figure 1.1 is a map of the basin. <br />1.1. Physical Geography <br />The Upper Colorado River basin is approximately 9,916 square miles in size (excluding the <br />Gunnison River basin). It ranges in elevation from 12,800 feet at its headwaters to 4,325 feet near <br />the Colorado-Utah state line. The Upper Colorado River is the primary stream in the basin, with <br />major tributaries including the Fraser River, Williams Fork River, Muddy Creek, Blue River, Eagle <br />River, Roaring Fork River, Rifle Creek, and Plateau Creek. The Gunnison River enters the Colorado <br />River downstream of the major projects and users on the Colorado River. Average annual <br />streamflow in the upper drainage (USGS gage near Grand Lake, Colorado) is approximately 57,000 <br />acre-feet, which increases to an annual average of 4.9 million acre-feet below Grand Junction, <br />Colorado (USGS gage near the state line), including the Gunnison River inflows, for water years <br />1975 to 2005. The water rights of the Gunnison River basin are not included in the Upper Colorado <br />River Model; rather the Gunnison River is treated as a gaged inflow in the Upper Colorado River <br />Model at USGS gage 09152500. <br />1.2. Human and Economic Factors <br />The area remains moderately populated, with the 2000 census estimates placing the combined <br />populations of Eagle, Garfield, Grand, Mesa, Pitkin, and Summit Counties at 252,567. Grand <br />Junction and Glenwood Springs are the major population centers in the basin, with approximately <br />42,000 and 7,700 residents in 2000 and growth rates of 45 percent and 18 percent, respectively, from <br />1990 to 2000. Summit and Eagle Counties grew over 83 percent and 90 percent, respectively, from <br />1990 to 2000. Modest population growth was experienced in Pitkin and Mesa Counties at 17 percent <br />and 25 percent, respectively, over the 1990 to 2000 period. The towns of Aspen and Vail <br />experienced 25 and 28 percent growth, respectively, over the 1990 to 2000 time period. Population <br />growth was generally concentrated in the lower portions of the basin at the existing maj or population <br />centers. Growth was seen in the upper portions of the basin at a more modest pace. This attests to the <br />continued importance of recreation-based activities, as the ski areas and other outdoor recreation <br />opportunities draw people and increase tourism within the basin. <br />The major water use in the basin is irrigation, with several thousand irrigation ditches diverting from <br />the mainstem and the numerous tributary streams throughout the basin. Diversions from many of the <br />small irrigation ditches average one or two thousand acre-feet per year. There are also several larger <br />irrigation ditches, such as the Government Highline Canal which diverts approximately 770,000 <br />Upper Colorado River Basin Information 1-1 <br />