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l1 ~ ~ ~ l n ~ ~ ~ lr Doc Date:1211112001 II I I II I II i II IIII III In closer proximity to the Nucla Mine, Figure 7-22 and Figure 7-23 are histograms of total • monthly flows taken at the USCS gaging stations during the 1977 water year for the San Miguel River at Naturita, Colorado and the San Miguel River at Uravan, Colorado, respectively. Both histograms reflect the same basic annual pattern of streamflow as shown in Figure 7-21 for the Colorado River, indicating that, since 1957, man's activities have done little to alter the basic flow pattern. The San Miguel River at Naturita, Colorado has a drainage area of 1,080 square miles. lorns et al. (1965a) presents a flow-duration curve developed for the San Miguel River at Naturita based on historic streamflow records. The curve indicates that 90 percent of the time, discharge exceeds 60 cfs, and that flows larger than 1000 cfs occur only 10 percent of the time. The mean discharge of the San Miguel River at Naturita is 351 cfs. Regional Surface Water Quality. The geologic formations and soils over which the water flows greatly influences the chemical quality of surface water. In the Grand Division of the Upper Colorado River Basin, and more proximate, within the subdivision between the Gunnison and Green Rivers, this statement has been demonstrated by lorns et al. (1965a). • The San Miguel River Basin, into which Tuttle and Calamity Draws flow, originates in the San Juan Mountains which are principally composed of tertiary volcanic rocks. The lower altitude, interior portions of this basin are chiefly underlain by the Dakota Sandstone, the Morrison Formation and the Mancos Shale. Of these rocks, the Morrison Formation and the Mancos Shale have the greatest potential for influencing water chemistry through solution of readily available minerals. According to lorns (et al. 1965a), at times of high flow generated largely by snowmelt runoff from the upland areas underlain by volcanic rocks, the water is of the calcium bicarbonate sulfate type. During low flow, the water contains less bicarbonate and greater amounts of calcium and sulfate. As flow progresses downstream, the water quality will feature higher percentages of magnesium, potassium, sulfate and chloride. Generally, Total Dissolved Solids (TDS) varies inversely with discharge, and also increases in the downstream direction. lorns et al. (1965a) mentions that streams flowing from the Uncompaghre Plateau and • entering the Dolores River between the San Miguel River and the mouth ,of the Dolores commonly exhibit a calcium bicarbonate water type with an average TDS of less than 300 7-103 Revised 04/11/88