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. fH)liH8 W, W, WHEELER AND ASSOCIATES, INC. WATER RESOURCES ENGINEERS 3700 SOUTH INCA ENGLEWOOD, COLORADO 80110 PHONE 761.4130 . PART I. SUMMARY . McElmo Creek drains a 346 square mile area near the southwestern corner of Colorado and discharges into the San Juan River at. a point about 11 miles west of the Colorado-Utah state line. The general location of the McElmo Creek drainage basin is shown on Figure 1. The stream flows from east to west, with elevations within the drainage basin ranging from 7000 feet in the eastern areas to 4900 feet at the state line. Annual precipitation in the basin averages about 13 inches. . . The northern and eastern portions of the McElmo Creek drainage basin include much of the area known as the Montezuma Valley, which is dominated by irrigated farmland. An average of about 116,000 acre-feet of water per year is imported into the Montezuma Valley from the Dolores River, and an additional diversion of about 14,000 acre-feet per year will be provided by the recently completed Dolores Project. Most of the historic streamflow of McElmo Creek, averaging 33,000 acre-feet per year, is derived from irrigation return flows. . . The virgin flow of McElmo Creek is equal to the sum of the following: (1) short-term runoff in response to extreme precipitation events, (2) spring snowmelt runoff, and (3) base flow associated with general precipitation through- out the year, less stream losses associated with evaporation and riparian vegeta- tion. Short-term runoff was estimated through examination of the daily flow records of McElmo Creek at the USGS gaging station near the Colorado-Utah state line. It was found that the volume of short-term runoff averaged 6,300 acre-feet per year during the 1951 through 1980 study period. Base flow and snowmelt runoff ",as calculated through use of streamflow estimates for the lower reaches of the Mancos, La Plata, and Piedra River drainage basins. Stream losses on McElmo Creek in the McElmo Canyon area west of Cortez were esti- mated to be 3,200 acre-feet per year, but most of this stream loss may be associ- ated with irrigation return flows from Dolores River diversions. Taking these . . . -1- . ;j{ -'~"