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"., ~ ....;f ('''' Geological Survey with data collected during this study. In general, the variation was no more than 20 percent. The data were collected in this study following an abnormally dry year; thus, the calculated salinity contributed to the streamflow by ground water may be smaller than the long-term average. Although the assumption is made that the ground-water discharge is from natural sources in several areas, such as the Grand Valley, lower Gunnison, and lower Green River reaches, there may be an unquantified amount of irrigation return flow adding to the base-flow salinity load. Specific conductance and streamflow measurements were made at all sites, and samples were collected for chemical analyses at 78 of the sites. The chemical analyses are presented in table 11 (see Supplemental Information at back of report). A regression analysis of specific conductance versus laboratory-determined dissolved-solids concentration was performed for each basin in which chemical-analysis samples were taken. The results of this regression analysis were used to calculate the dissolved solids at sites where only specific conductance was measured. Available data were used for all other parts of the study area. In some areas, data were available from local and regional studies of ,the Upper Colorado River Basin (Iorns and others, 1964, 1965; Price and Arnow, 1974). In the Yampa River basin, extensive measurements of the quantity and quality of streamflow were made as part of a 3-year river-basin assessment (Steele and others, 1979). Extensive streamflow and quality of water data also were available for the upper Green River (Lowham and others, 1976). Discharge and water-quality data obtained at streamflow-gaging stations operated by the U.S. Geological Survey also were used. Calculation of the mass-balance equation. salt load transported in base flow was made using a The salt load was determined by: S=ACQ (1) where: S = salt load, in tons per year; A = conversion factor = 0.985; C = dissolved-solids concentration of base flow of the stream, in milligrams per liter; and Q = base-flow discharge of the stream, in cubic feet per second. The computed base-flow salt loads are based on the assumption that point measurements at various sites are representative of the mean ground-water contribution to the total annual salt load. Adjustments for reservoir effects were made to computed base-flow salt loads in some of the subbasins. ., The reservoir adjustments for both discharge and salt load were computed ~ by setting the outf10w,discharge and salt load equal to the measured inflow , and salt load. The adjusted dissolved-solids concentration is calculated using equation 1 rewritten as: 14