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<br />Tomichi Creek <br /> <br />~ <br />OJ The Tomichi Creek subbasin includes the area drained by Tomichi Creek and <br />~ tributaries (fig. 10). Igneous and metamorphic rocks underlie most of the <br />~ subbasin, but the Mancos Shale and Dakota Sandstone are present in a few <br />areas. <br /> <br />Three sampling sites were selected in this subbasin (figs. 10 and 13; <br />table 3). Dissolved-solids concentration measured at the three sites (4-6) <br />ranged from 167 to 195 mg/L. These values are slightly higher than would be <br />expected in a subbasin predominantly underlain by igneous and metamorphic <br />rocks. There are no water-quality stations in the Tomichi Creek subbasin for <br />comparison of historical data with sample data. The estimated base-flow salt <br />load from the Tomichi Creek subbasin was about 14,100 ton/yr at a measured <br />discharge of 78 ft'/s. <br /> <br />Upper Gunnison River <br /> <br />This subbasin includes the drainage area of the Gunnison River upstream <br />from the confluence with the North Fork Gunnison River but excluding the areas <br />contained in the East and Taylor Rivers and Tomichi Creek subbasins (fig. 10). <br />Igneous and metamorphic rocks are adjacent to the main stem of the Gunnison <br />River. The upper reaches of most of the tributary streams drain mostly <br />volcanic rocks. Large areas of Mancos Shale and Dakota Sandstone underlie the <br />west end of the subbasin. <br /> <br />Streamflow was sampled at 13 sites in this subbasin (figs. '10 and 13; <br />table 3). Dissolved-solids concentration in the Gunnison River near Gunnison <br />(site 7) was about 120 mg/L at a measured base-flow discharge of 207 ft'/s. <br />Most of the flow at this site comes from the East and Taylor River'drainages. <br />Downstream from this site, the Gunnison River is controlled by Blue Mesa and <br />Morrow Point Reservoirs. Eleven tributary streams discharge directly into <br />Blue Mesa or Morrow Point Reservoirs. Seven of these were sampled: Beaver <br />Creek (site 8), Cebolla Creek (site 9), Lake Fork Gunnison River (sites 10 <br />and 11), East Elk Creek (site 12),' Soap Creek (site 13), Curecanti Creek <br />(site 14), and Big Blue Creek (site 15). The dissolved-solids concentration <br />at these sites ranged from about 60 to 123 mg/L. The discharge of these seven <br />tributaries ranged from 3.9 to 39 ft'/s. The total combined discharge was <br />99 ft'/s, and the combined base-flow salt load was 9,100 ton/yr. These <br />tributaries drain primarily igneous and metamorphic rocks in the lower reaches <br />and volcanic rocks in the upper reaches. Most of the flow in these streams is <br />probably from the volcanic rocks. <br /> <br />The Lake Fork of the Gunnison River was sampled near its mouth and also <br />in the headwaters region near Lake' City, Colo. Most of the flow of the Lake <br />Fork of the Gunnison River was produced in the headwaters region, which <br />contains predominantly volcanic rocks. Combined discharge of the four streams <br />not sampled that discharge directly into the reservoirs was estimated at <br />15 ft'/s adding 1,400 ton/yr base-flow salt load based on the results measured <br />at the other seven sites. <br /> <br />39 <br />