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<br />lJOHJoO <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />PARTVll. VIRGm STREAMFLOW ANALYSIS <br /> <br />An analysis of virgin streamflow is normally made through use of historic <br /> <br /> <br />streamflow gage records, with appropriate adjustments to account for stream <br /> <br /> <br />depletions associated with ilTigation, municipal use, reservoir storage, and trans- <br /> <br /> <br />basin diversions. This general approach was used in the virgin streamflow analyses <br /> <br />made by W. W. Wheeler and Associates, Inc. for the Mancos River, La Plata <br /> <br /> <br />River, Animas River, Florida River, Piedra River, and the San Juan River, as <br /> <br /> <br />described in the hydrology analysis report for each river. Conditions in the <br /> <br /> <br />McElmo Creek drainage basin are unusual and preclude the utilization of an <br /> <br /> <br />approach where historic streamflow records are adjusted to derive virgin stream- <br /> <br /> <br />flow estimates. Approximately 70 percent of the McElmo Creek historic stream- <br /> <br /> <br />flow results from ilTigation return flows from the imported diversions of Dolores <br /> <br /> <br />River water for ilTigation in the Montezuma Valley. The native flows of McElmo <br /> <br /> <br />Creek are intermingled with the imported water for ilTigation. Overall ilTigation <br /> <br /> <br />efficiency, crop consumptive use, and return flow lag patterns cannot be esti- <br /> <br /> <br />mated with sufficient accuracy to provide usable estimates of native virgin <br /> <br /> <br />stream flows. As an additional complicating factor, return flows from Dolores <br /> <br /> <br />River diversions enter Yellow Jacket Creek and Navajo Wash as well as McElmo <br /> <br /> <br />Creek, but no reliable measurements are available of the amounts of return <br /> <br /> <br />flows that enter each stream. Therefore, even if ilTigation depletions in the <br /> <br /> <br />Montezuma Valley could be accurately estimated on a monthly basis, data is <br /> <br /> <br />not available to estimate how much of the return flows from Dolores River <br /> <br /> <br />diversions have historically entered each of the three stream systems. <br /> <br />The alternate method of analysis that was adopted for calculation of <br /> <br /> <br />the McElmo Creek virgin flows includes the following: <br /> <br />A. <br /> <br />Runoff contributions per square mile from the lower reaches of <br />the Mancos, La Plata, and Piedra River basins were calculated. <br />On this basis, the total virgin streamflow of McElmo Creek at <br />the State Line gaging station, including runoff from short-term <br />precipitation events, was estimated to average 26 acre-feet per <br />square mile per year. <br /> <br />-13- <br /> <br />; ~-,;",- <br />