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<br />. <br /> <br />. <br /> <br />. <br /> <br />OOii2J3 <br /> <br />CHAPTER V <br /> <br />WATER RESOURCES <br /> <br />Isolines of mean annual runoff for the 1920-59 period, taken from that <br />report, appear on the map on page 45. <br /> <br />The isolines indicate that the average annual yield of the 350- <br />square-mile drainage area of the La Sal Mountains is about 100,000 acre- <br />feet and that of the 780-square~mile area of the Abajo Mountains, includ- <br />ing Elk Ridge, about 140,000 acre-feet. About 77 square miles of the <br />La Sal Mountain Area and 37 square miles of the Abajo Mountain Area are <br />9,000 feet or more in elevation. <br /> <br />Runoff from the 7,830 square miles of the study area that is outside <br />of these mountain areas, most of which is below 7,000 feet in elevation, <br />is estimated at only about 5 to 10 percent of the precipitation. The an- <br />nual runoff would thus average about 0.6-inch depth from the large area <br />of these lower lands, or 250,000 acre-feet. Only a small portion of this <br />water is usable because of its erratic flow, high sediment loads, and the <br />rugged terrain. <br /> <br />Storage Reservoirs <br /> <br />Except for Lake Powell on the Colorado and San Juan Rivers, storage <br />reservoirs are relatively few and of small size in the study area, Lake <br />Powell, located below all area lands, does not contribute to the water <br />supply but does have great recreational value. Unfavorable topographic <br />conditions on the mountain watersheds within the study area above places <br />of water use have limited reservoir construction. ' <br /> <br />Only 11 reservoirs within <br />feet or more. Data on these <br />addition to these reservoirs, <br />acre-foot capacity have been <br />ing domestic and stock water, <br />life. <br /> <br />the study area have capacities of 100 acre- <br />are tabulated on the following page. In <br />numerous impoundments of less than 100- <br />constructed for various purposes, includ- <br />irrigation, recreation, and fish and wild- <br /> <br />Grow1d Water <br /> <br />The principal ground water aquifers in the San Juan Area are in bed- <br />rock or consolidated formations which ~~derlie much of the area. Allu- <br />vium or other unconsolidated materials are not known to constitute an <br />important ground water source except in Spanish Valley and possibly in <br />Castle Valley, both of which extend northwest from the La Sal Mountains <br />toward the Colorado River. <br /> <br />48 <br />