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<br />002253 <br /> <br />Chapter I <br /> <br />Introduction <br /> <br />Elevations in the Gunnison River Basin range from approximately 1410 meters (4628 feet) to <br />4350 meters (14.270 feet). The lower portion of the basin consists of wide, relatively llat valleys. <br />These valleys are flanked by mesas with steep slopes connecting the mesas to the valleys. The <br />upper portion of the basin is characterized by high, glaciated peaks with steep slopes that drain <br />down to subalpine basins. <br /> <br />Climate in the Gunnison River Basin varies considerably. The lower portion of the basin is <br />semiarid, in places receiving less than 25 centimeters (\0 inches) of precipitation per year. The <br />upper portion of the basin. due to orographic effects, is much wetter. Areas in the Elk Mountains <br />in the northeastern portion of the basin receive, on average, over 100 centimeters (40 inches) of <br />precipitation per year. Most of the precipitation in the basin falls as snow, and the vast majority <br />of the streamflow in the basin originates as snowmelt in the mountainous headwaters. <br /> <br />Because streamflow is dominated by snowmelt, the natural flow of waters in the Gunnison River <br />Basin is very high in the spring. diminishing rapidly in midsummer. While summer and fall flows <br />can be augmented by rain events, natural flows in the summer, fall, and winter months are <br />generally low. The average annual discharge of the Gunnison River Basin is approximately <br />2.31 billion cubic meters (1.87 million acre-feet).' <br /> <br />Reservoirs in the Gunnison River Basin (table I) modify the natural flow pattern. allowing a more <br />uniform flow of water to occur throughout the year. <br /> <br />There are approximately 107,000 hectares (264,000 acres) of irrigated land within the Gunnison <br />River Basin. Water is diverted from the rivers and creeks in the basin to irrigate these lands. <br />Approximately 600 million cubic meters (500,000 acre-feet) of water is consumed annually in the <br />basin.) Most of the irrigation relies on upstream storage of water in reservoirs to assure a supply <br />of water throughout the growing season. In areas where irrigation occurs from the natural flow of <br />unregulated tributaries. late season shortages are common. <br /> <br />Z Source: "Gunnison River at Gnmd Junction." USGS gage u!.ing data from 1897-1993. <br /> <br />1 ThiS figure was obtained by subtracting historic nows from natural flows (as conlained in the Colorado River <br />Simulmion System natural now database) for the Gunnison River at Grand Junclion. Colorado. <br /> <br />3 <br />