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...'" a;. -.1 o aquifers, the water has time to dissolve soluble mineral constituents. Most igneous and metamorphic rocks are composed primarily of silicate minerals, such as quartz, that are not readily 'soluble. Water from these rocks generally contains few dissolved solids. Some sedimentary rocks in the basin, primarily shales of marine or lacustrine origin, contain large amounts of readily soluble minerals. The abundance of soluble minerals, in conjunction with the small permeability of these shales, results in large concentrations of dissolved solids in the water. Coarse-textured sedimentary rocks, such as sandstones, contain fewer soluble minerals and have relatively large permeabilities. Therefore, water in these rocks generally contains fewer dissolved solids than water in shales. I I A list of the major hydrogeologic units in the Upper Colorado River Basin is given in table 1. The potential that each geologic formation has for contributing saline waters is presented in tahle 1. The general locations of the major hydrogeologic units within the Upper Colorado River Basin is shown in figure 2. Precambrian rocks underlie the headwaters of most of the major streams in the study area. The dissolved-solids concentration of this water is very small, generally less than 100 mg/L (milligrams per liter), but because the volume of water coming in contact with the Precambrian rocks is so great, they contribute most of the salt to streams in the study area. Some formations, such as the Paradox Member of the Hermosa Formation of Pennsylvanian age, discharge very small quantities of extremely saline ground water and produce large salt loads downstream. The sources of these mineralized waters are black shale containing interbedded anhydrite and dolomite and beds of potash salt and halite. The sedimentary formations that contribute most significantly to the salinity level (dissolved-solids concentration) of the Colorado River are Upper Cretaceous marine shales in hydrogeologic unit 7 (table 1). These shales normally discharge small quantities of moderately saline ground water. Soluble salts dissolved from the marine shales by overland runoff and by eroding streams that cross extensive outcrops of the shales cause the largest increase in salinity. DATA COLLECTION AND METHOD OF ANALYSIS Discharge and water-quality data were collected at 142 sites in the Upper Colorado River Basin upstream from the confluence of the Colorado and Green Rivers in December 1977 and January 1978. The data-collection sites were located so that the salt load from individual aquifer groups could be identified, thus allowing the results of this study to be more readily transferred to other areas. A one-time sampling program was conducted. The assumption was made that the ground-water discharge from aquifers remains nearly constant during the year and also from year to year. That is, the variation of the ground-water discharge to streams during the year is assumed to be minimal, but no calculation was made to verify this. The year-to-year variation of ground-water discharge to the streams was evaluated by comparing base-flow hydrographs from streamflow-gaging stations operated by the U.S. 12