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<br />'-" <br />a> <br />~ <br />~ <br /> <br />of base-flow salt load at a measured discharge of 63 ft3/s. This fairly large <br />salt load is produced primarily in the lower reaches of the subbasin by the <br />Mancos Shale. This subbasin alone contributed a larger estimated base-flow <br />salt load per year than the combined estimated base-flow salt load from East <br />and Taylor Rivers, Tomichi Creek, and upper Gunnison River subbasins. The <br />subbasin had a measured base-flow discharge of only 14 percent of the combined <br />base-flow discharge from those three subbasins. <br /> <br />UncompahRre River <br /> <br />This subbasin includes the drainage area of the Uncompahgre River <br />(fig. 10). The headwaters areas of the subbasin are underlain primarily by <br />volcanic rocks. The remainder of the subbasin is underlain primarily ,by <br />sedimentary rocks. Upland areas along the western part of the subbasin are <br />generally Dakota Sandstone. In the eastern part of the,subbasin, upland areas <br />are underlain principally by glacial till and other unconsolidated rocks of <br />Quaternary age. The subbasin at lower altitudes is underlain primarily by <br />large areas of the Mancos Shale. <br /> <br />Nine sampling sites were selected in the Uncompahgre River subbasin <br />(figs. 10 and 14; table 3). Dallas Creek (site 25), Cow Creek (site 26), the <br />Uncompahgre River at Colona (site 27), and Horsefly Creek (site 28) had <br />dissolved-solids concentrations ranging from about 440 to about 710 mg/L. The <br />drainage basins of all of these streams are, underlain by the Mancos Shale, <br />which is probably responsible for the relatively large dissolved-solids <br />concentrations. Dissolved-solids concentrations at the five remaining sites <br />(29-33) in the subbasin ranged from about ,1,100 to about 2,300 mg/L. All <br />these drainages are underlain by extensive deposits of Mancos Shale and also <br />may be influenced by the residual effects of extensive irrigation within the <br />basin. <br /> <br />A water-quality station is located on the Uncompahgre River at Delta, <br />Colo., (site 33). Average dissolved-solids concentration at this station for <br />December, January, and February of water year 1977 was 1,760 mg/L, which is <br />only a 3-percent difference from the measured value of 1,820 mg/L at this <br />site. The Uncompahgre River subbasin contributed an estimated base-flow salt <br />load of 323,000 ton/yr and had a base-flow discharge of 180 ft3/s. The <br />drainage area upstream from Colona, Colo., (site 27) produced an estimated <br />, base-flow salt load of 56,000 ton/yr, which represents about one-sixth of the <br />total estimated base-flow salt load from the subbasin. The drainage area <br />between Co10na and the mouth of the Uncompahgre River produced an estimated <br />base-flow salt load of about 267,000 ton/yr and a discharge of 90 ft3/s. <br />Estimates using measured tributary inflow accounted for only 138,200 ton/yr of <br />the estimated base-flow salt load but accounted for all of the additional <br />discharge. This indicates that direct channel erosion of the Mancos Shale by <br />the Uncompahgre River may have produced an estimated 129,000 ton/yr of <br />base-flow salt load. The Uncompahgre River subbasin contributed an estimated <br />2.4 times more base-flow salt load than the combined estimated base-flow salt <br />load of the four other previously discussed subbasins. <br /> <br />~ <br /> <br />41 <br />