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<br />. <br /> <br />f;1-. -, R"'" <br />')., 0 <br />~1, J t. \.< .- <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />associated with snowmelt runoff and general precipitation throughout the year. <br /> <br /> <br />River losses occur as a result of evaporation from the river and evapotranspiration <br /> <br /> <br />from riparian vegetation along the river. <br /> <br />The calculated difference between the estimated virgin flow at the state <br /> <br /> <br />line and the recorded historical streamflow is attributable to depletion of the <br /> <br /> <br />river associated with irrigation practices. Diversion records and irrigation <br /> <br /> <br />practices were analyzed and estimates of historic stream depletion were prepared. <br /> <br />A comparison was made to confirm that the virgin flow estimate at the state <br /> <br /> <br />line was consistent with the historical irrigation depletion and streamflow records. <br /> <br /> <br />Although historical flows at the state line gage have averaged about 25,100 <br /> <br /> <br />acre-feet per year, the virgin flows were estimated to average 41,200 acre-feet <br /> <br />per year, reflecting a historical irrigation depletion of about 16,100 acre-feet. <br /> <br /> <br />Virgin stream flows at the state line were found to vary from a low of about <br /> <br /> <br />,7,800 acre-feet in 1977 to a high of about 92,600 acre-feet in 1980. <br /> <br />Gage records are available for Cherry Creek, a major tributary of the <br /> <br /> <br />La Plata River, for the period 1938 through 1950. The natural flow of Cherry <br /> <br /> <br />Creek is influenced by diversions into the watershed by the La Plata River and <br /> <br /> <br />Cherry Creek Ditch and by irrigation of about 1,700 acres through use of the <br /> <br /> <br />imported and native water supplies for irrigation. The virgin flow of Cherry <br /> <br /> <br />Creek was estimated through reference to the available flow records and an <br /> <br /> <br />irrigation depletion analysis for the Cherry Creek watershed. More than 50 <br /> <br /> <br />percent of the inflow to the La Plata River downstream of Hesperus was found <br /> <br /> <br />to be contributed by Cherry Creek. The virgin flow of Cherry Creek was esti- <br /> <br /> <br />mated to average 6,500 acre-feet per year, varying from a low of 1,300 acre-feet <br /> <br />in 1956 to a high of 21,400 acre-feet in 1973. <br /> <br />The report includes a despiption of the methodology used in calculating <br />the virgin flow of the La Plata River. Summaries of basic data used in the <br />analysis are included. <br /> <br />-2- <br /> <br />.s ..,_-~ <br />