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<br />~ <br />~ <br />~ <br />~ <br /> <br />A plot of salt load, dissolved-solids concentration, and discharge for <br />the main stem of the Gunnison River (fig. 15) graphically depicts the impact <br />of these sources on the salinity levels of the Gunnison River. Little change <br />in salt load or dissolved-solids concentration is apparent until the <br />confluence with the North Fork Gunnison River. Downstream from there the <br />trend toward a sharp increase in dissolved-solids concentration and salt load <br />is apparent. The most apparent impact is the large increase in salinity <br />levels of the Gunnison River by the addition of the Uncompahgre River. <br /> <br />A comparison of the estimated base-flow salt load of 724,000 ton/yr with <br />the total annual salt load of 1,364,600 ton/yr reported by BLM (Bentley and <br />others, 1978) indicates that about 53 percent of the total estimated annual <br />salt load for the Gunnison River basin is contributed by ground-water sources. <br /> <br />Colorado Lower Headwaters Subregion <br /> <br />The Colorado lower headwaters subregion consists of the drainage area of <br />the Colorado River between approximately the Colorado-Utah State line and <br />Glenwood Springs but excluding the Gunnison River basin. The subregion has' <br />a drainage area of about 3,800 mi2 (fig. 1). Average annual precipitation <br />ranges from less than 8 to more than 40 in. <br /> <br />West of G1enwood Springs, the Colorado River flows through a relatively <br />low-lying arid region. Most of the smaller tributaries in this reach are <br />ephemeral. Between Glenwood Springs and Grand Junction, the largest tributary <br />to the Colorado River is Plateau Creek, which has a mean' annuai discharge of <br />about 130,000 acre-ft or about 100 acre-ft/mi2. The mean annual dissolved- <br />solids concentration of Plateau Creek is approximately 340 mg/L. 'At Grand <br />Junction, the Gunnison River joins with the Colorado River. The Colorado <br />River at the Colorado-Utah State line has a mean annual discharge of about <br />4.3 million acre-ft and a mean annual dissolved-solids concentration of about <br />600 mg/L. <br /> <br />The Colorado lower headwaters subregion is underlain principally by <br />Tertiary sandstone, mudstone, claystone, and shale of the Wasatch and Green <br />River Formations. Oil shale is present in the Green River Formation. <br />Cambrian, Ordovician, Devonian, and Mississippian rocks, and exposures of the <br />Dakota Sandstone, Mancos Shale, Mesaverde Group, and related formations are <br />found in the Grand Hogback. The Grand Valley near Grand Junction consists <br />mainly of Mancos Shale. <br /> <br />Measurements of specific conductance and stream discharge at 19 sites <br />were made in the Colorado lower headwaters subregion (fig. 16). Samples were <br />collected for chemical analyses at five sites. Dissolved-solids <br />concentrations for the remaining 14 sites were calculated using a linear <br />regression analysis (fig. 17) of specific conductance measured at these five <br />sites versus dissolved-solids concentration determined in the laboratory. <br />Values of discharge, specific conductance, dissolved-solids concentration, and <br />salt load for each site are presented in table 4. The subregion was not <br />divided into any subbasins. The data for sites in the Colorado lower <br />headwaters subregion are shown in figures 16 and 18 and in table 4. <br /> <br />44 <br />