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1. The Gunnison River Basin <br />The Gunnison River basin extends from the Continental Divide to Grand Junction, where it joins the <br />Colorado River. The basin encompasses all of Gunnison, Delta, and Ouray counties, and parts of <br />Montrose, Saguache, Hinsdale, and Mesa counties in Colorado. Figure 1.1 is a map of the basin. The <br />Gunnison River and its largest tributary, the Uncompahgre River, flow through forested mountains and <br />rural irrigated valleys. <br />1.1. Physical Geography <br />The Gunnison River basin is approximately 7,800 square miles in size, ranging in elevation from <br />14,000 feet in the headwaters to 4,550 feet at Grand Junction. Across this expanse, average annual <br />rainfall varies from more than 40 inches in the high mountains to as little as 8 inches in the <br />Uncompaghre Valley near the Town of Delta. Temperatures generally vary inversely with elevation, <br />and variations in the growing season follow a similar trend. The Town of Gunnison has an average <br />growing season of 144 days, while the growing season at Grand Junction has been estimated at <br />approximately 228 days. <br />The Gunnison River begins at the confluence of the East and Taylor Rivers, about 10 miles upstream <br />from the Town of Gunnison. The flow increases as Cochetopa and Tomichi Creeks join the river <br />near the Town of Gunnison. Just downstream, the river has carved through Precambrian rocks to <br />form the Black Canyon of the Gunnison. Annual flow through the Town of Gunnison is 547,000 <br />acre-feet per year (United States Geological Survey [USGS] gage near Gunnison). <br />The Uncompahgre River is the largest <br />tributary to the Gunnison River, entering <br />from the south near the Town of Delta. <br />Average annual flow of the <br />Uncompahgre near the confluence is <br />220,000 acre-feet (USGS gage at Delta). <br />The average annual flow of the <br />Gunnison River near Grand Junction is <br />over 1.8 million acre-feet (USGS gage <br />near Grand Junction). Approximately 60 <br />percent of this flow is attributable to <br />snowmelt runoff in May, June, and July. <br />Gunnison River Basin Information 1-1 <br />Gunnison River at Black Canyon National Park <br />