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~V';"l3t8 c.-,J..) ,;. . . . Project by the Bureau of Reclamation. These gages provide a period of record of 1938 through 1951. The virgin streamflow at the confluence was determined by correcting the records for irrigation diversions and return flows upstream of the gages. Missing flow data for 1952 through 1980 were estimated through correlation with gage records for the La Plata River near Hesperus that span the entire period of 1938 through 1980. Since the upper La Plata and Mancos river basins are adjacent, and both rivers originate in the La Plata Mountains, the hydrological characteristics of the two rivers were found to be almost identi- cal, resulting in a good correlation between the respective gage records. The virgin flows of the Mancos River at locations downstream of the conflu- ence of the West, Middle, and East Mancos rivers were calculated as being equal to the virgin flow at the confluence plus inflow contributions to the river down- stream of the confluence, less river losses. Inflows consist of two components: (1) short-term runoff that occurs in response to precipitation from thunderstorms during summer and fall and, (2) other river gains and base flow associated with snowmelt runoff and general precipitation throughout the year. River losses consist of evaporation from the river and evapotranspiration from riparian vegeta- tion along the river. A gaging station has been maintained by the U.S. Geological Survey (USGS) at a location on the Mancos River near the bridge at Highway 666, south of Towaoc. The calculated difference between the estimated virgin flows at this station and the recorded historic streamflow is attributable to depletion of the river associated primarily with irrigation and municipal water use. Diversion records and irrigation practices were analyzed and estimates of historic stream depletion were prepared. A comparison was made to confirm that the virgin flow estimate at the Highway 666 gage location was consistent with the historic stream depletion and historic gage records. Although historic flows at the gage near Highway 666 have averaged 3l,l00 acre-feet per year, the virgin flows were estimated to average 50,200 acre-feet per year, reflecting an historic irrigation depletion of about 19,100 acre-feet. Virgin flows were found to vary from a low of about 9,000 acre-feet in 1977 to a high of about IlI,OOO acre-feet in 1973. -2- i,;