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Histograms of total monthly flows taken at the USGS gaging stations during the 1977 water year <br />for the San Miguel River at Naturita, Colorado and the San Miguel River at Uravan, Colorado, <br />respectively reflect the same basic annual pattern of streamflow as shown in Peabody Figure 7-21 <br />for the Colorado River, indicating that, since 1957, man's activities have done little to alter the basic <br />flow pattern. <br />The San Miguel River at Naturita, Colorado has a drainage area of 1,080 square miles. lorns et <br />a1. (1965a) presents aflow-duration curve developed for the San Miguel River at Naturita based <br />on historic streamflow records. The curve indicates that 90 percent of the time, discharge exceeds <br />60 cfs, and that flows larger than 1000 cfs occur only 10 percent of the time. The mean discharge <br />of the San Miguel River at Naturita is 351 cfs. <br />Regional Surface Water Quality. The geologic formations and soils over which the water flows <br />greatly influences the chemical quality of surface water. In the Grand Division of the Upper <br />Colorado River Basin, and more proximate, within the subdivision between the Gunnison and <br />Green Rivers, this statement has been demonstrated by lorns et a1. (1965a). <br />The San Miguel River Basin, into which Tuttle and Calamity Draws flow, originates in the San Juan <br />Mountains which are principally composed of tertiary volcanic rocks. The lower altitude, inferior <br />portions of this basin are chiefly underlain by the Dakota Sandstone, the Morrison Formation and <br />the Mancos Shale. Of these rocks, the Morrison Formation and the Mancos Shale have the <br />greatest potential for influencing water chemistry through solution of readily available minerals. <br />According to lorns (et a1. 1965a), at times of high flow generated largely by snowmelt runoff from <br />the upland areas underlain by volcanic rocks, the water is of the calcium bicarbonate sulfate type. <br />During low flow, the water contains less bicarbonate and greater amounts of calcium and sulfate. <br />As flow progresses downstream, the water quality will feature higher percentages of magnesium, <br />potassium, sulfate and chloride. Generally, Total Dissolved Solids (TDS) varies inversely with <br />discharge, and also increases in the downstream direction. <br />lorns et a1. (1965a) mentions that streams flowing from the Uncompaghre Plateau and entering <br />the Dolores River between the San Miguel River and the mouth of the Dolores commonly exhibit <br />Revised 25 Jul 2002 2.04.7-21 <br />