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I I I I I I I I , I I I I I I I I I I I ...,..'<lIIt~.l'<'-flll., OJ~.zh4 Chapter 2 - Oefinll Water Supply & Water Quality Needs Section 2 - DESCRIPTION OF STUDY AREA AND WATER USE General The Study area is the entire East River drainage of west-central Colorado, which is a portion of the Gunnison River Basin. This area is located entirely within Gunnison County. The East River, a tributary of the Gunnison River, collects water from str~ams including the Slate River, Ceml~nt Creek, Coal Creek, Washington 3ylch, Copper Creek, Oh-Be-Joyful Creek and Brush Creek. After joining its primary tributary, the Slate River, the East River flows in a southeasterly direction to Almont. Here it joins the Taylor River to form the Gunnison River. StreamflolVs in th;" region are generally highest in the late spring and early summer when the winter snowpack is melting. In the late summer and into the winter, streamflows decrease to annual lows. Table I displays monthly virgin flow volumes for the East River basin from 1952 through 1990. ,Virgin flow is the flow which would have occurred in the absence of human activities, such as irrigation. Flow values were developed by taking the historical flows. at the Almont gage and adding the estimated irrigation consumptive ul/e upstream of the gage. Because municipal and domestic consumptive u$e is so small, it was disregarded in these calculations. The basin yield~d an average of 239,800 acre-feet of water during this 39 year period. A minimum annual discharge of 79,200 acre-feet occurred in 1977, and a maximum discl1arge of 420,900 acre-feet occurred in 1957. Figure 1 plots monthly mean, 'minimum and maximum values. As this figure shows, about 73 percent of the runoff occurs during 3 months of the year (May thru July), and almost 85 percent occurs in the 5 month period of April through August. Estimates of annual water use by various components of major human activity in the basin are displayed in Table 2. The irrigation component is by far the largest water user, diverting an estimated average of 88,600 acre-feet annually based upon 17-years Of diversion records maintained by the Colorado Division of Water Resources. Th~se records include most major ditches and report the diversions made througho!Jt the year. The Roaring Judy Fish Hatchery has diverted an average of 21,300 acre-feet annually over the last 3 years of record and is the second largest user; however this is primarily a non-consumptive use. Instream flow rights held by the Colorado Water Conservation Board in the East 49 .' .i;, .. n_ ,_,~' ",~_._ .." _..~. , ',i _,::~ ",,<, .c"""",,"'.~