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<br />0~:~~~ <br /> <br />. <br /> <br />Local reports indicate that considerable capacity in the <br />plains reservoirs has been lost through sediment accumulation and <br />deterioration of the old reservoir dikes and outlet works. <br /> <br />.., <br /> <br />Accurate information on the operation and efficiencies of these <br />plains reservoirs is unavailable. Except for a few records of reser- <br />voir releases, the only records are monthly contents based on gage <br />height readings. Only the contents of the larger reservoirs, about <br />28, are reported to the State Engineer. Their total combined <br />capacity is 147,000 acre-feet, or 91 percent of the total decreed <br />capacity. <br /> <br />The monthly reservoir contents from 1930 to 1949 were tabulated <br />for the South Platte Basin Report. This tabulation showed maximum <br />and minimum aggregate contents of the reported reservoirs of 133,800 <br />acre-feet and 5,100 acre-feet, respectively. The reservoir contents <br />of May 1 and November 1 were tabulated from the Water Commissioner's <br />reports for the 1947 to 1960 period. An estimated operation of the <br />plains reservoir~, considered as a group, was derived for the <br />1947-1960 study period by correlating the above-mentioned ta~~-~ions <br />with water supply conditions. The resulting average annua1jie1d of <br />the plains reservoirs "as 60,000 acre-feet for the 1947-1960'Period, <br />ranging from a minimum of 22,000 acre-feet in 1957 to a maximum of <br />90,000 acre-feet in 1947. <br /> <br />. <br /> <br />Eased on the length of feeder canals and other studies in the <br />South Platte Basin, canal losses were estimated to be 20 percent of <br />the diversions made from the river to storage. The average annual <br />net surface loss was estimated at 1.78 acre-feet per acre for the <br />plains reservoirs which have a total surface area of 10,000 acres. <br /> <br />The storage decrees of the p1s.ins reservoirs are junior to most <br />direct use rights on the Cache la Poudre. The major diversions to <br />storage occur during the spring runoff from flows in excess of the <br />diversions under direct flow rights. In the nonirrigation periods <br />the stream flow is generally diverted to storage as well as an <br />estimated 28 percent of the return flow. <br /> <br />The analysis of the plains reservoir operation indicates ma.x:l.mwn <br />storage genera.lly occurs by the end of June with the reservoirs being <br />drawn down as needed until the end of the irrigation season in <br />September. Ana.1ysis of past reservoir operations indicated a storage <br />carry-over of at least 20,000 acre-feet is desired for operations. <br />This practical minimum leye1 of storage also provides considerable <br />recrea~10n anlf IIshing VaLues. <br /> <br />. <br /> <br />Surplus Flows <br /> <br />In some years large quantities of water are still unregulated <br />in the Cache la Poudre River. Whenever the plains reservoirs are <br />filled, water in excess of the needs of the direct flow appropriations <br />passes out of the basin unused. <br /> <br />18 <br />