2020-06-04_PERMIT FILE - C1981008A (13)

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same basic annual pattern of streamflow as shown in Peabody 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. Iorns 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 Ouality 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 Iorns 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 Iorns (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. Iorns 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 ppm. These streams flow largely over the Glen Canyon Group and the Dakota Formation. Historic water quality samples taken at Naturita, Colorado indicate that the San Miguel River has a weighted average TDS of 316 ppm and an average TDS discharge of 101 tons per square mile. Specific conductance ranges between 318 and 730 umhos, and the mean Sodium Adsorption Ratio (SAR) is about 0.7. Based on these values, waters of the San Miguel at Naturita are classified as C2 -S 1, which categorizes the San Miguel River as being good for irrigation use. As of 1957, approximately 15,000 acres underlain by Dakota sandstone were actively irrigated between Placerville and Naturita, Colorado (Iorns, et al. 1965a). Using historic streamflow information, Iorns et al. (1965a) estimated a water budget for the San Miguel River in this vicinity, and concluded that the TDS contribution to the San Miguel River from irrigation is about 2.8 tons/year per acre of irrigated land. As of 1957, waters of the San Miguel River (except at the mouth during low flows) were suitable for domestic use, while during low flow, high levels of sulfate would render the water unsuitable. Page 2.04.7-16 March 2017 (TR -77)