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~ ~ Colorado River Region ~ .~ Most of the flow of the Colorado River originates on the western slope of the Rocky Mountains in Colorado. Areas in Utah contribute only minor amounts of the flow. The drainage area of the Colorado River in western Colorado and eastern Utah is about 27,000 mi2. The average flow of the Colorado River upstream from the confluence with the Green River is about 4.6 million acre-ft/yr (Bentley and othe'rs, 1978). The mean annual dissolved-solids concentration is about 610 mg/L, and the mean annual salt load is about 3.8 million tons. The Colorado River region was divided into five subregions: the Colorado, Colorado lower headwaters, Colorado upper headwaters, Gunnison, and' Dolores (fig. 1). Colorado Upper Headwaters Subregion The drainage area of the Colorado upper headwaters subregion is about 6,000 mi2 (fig. 1). The average annual precipitation above 9,000 ft altitude ranges from about 25 to about 50 in. and is mostly snow. Below 9,000 ft, the average annual precipitation ranges from about 12 to about 25 in. Measurements of specific conductance and stream discharge at 52 sites were made in the subregion (fig. 3). At 23 of the sites, samples were collected for chemical analysis. A linear regression (fig. 4) of specific' conductance measured at 22 of the sites versus dissolved-solids concentration determined in the laboratory was used to calculate the dissolved-solids concentration at the 29 nonmeasured sites. One site (site 52, table' 2) Colorado River below Glenwood Springs, Colo., was not included in the regression because localized impacts alter its ability to be representative of conditions farther up in the subregion. Values of discharge, specific conductance, dissolved-solids concentration, and salt loads for each site are presented in table 2. For purposes of discussion, the,Colorado upper headwaters subregion was divided into four subbasins: Blue River, Eagle River, Roaring Fork River, and the main-stem Colorado River upper headwaters (fig. 3). Blue River The Blue River subbasin includes the drainage area of the Blue River and tributaries upstream from the confluence with the Colorado River. The Blue River has its headwaters in a steep mountainous area south of the main stem of the Colorado River. Igneous and metamorphic rocks are exposed at higher elevations in the subbasin. At lower elevations, Upper Cretaceous shales in hydrogeologic unit 7 (table 1) crop out principally along the eastern side of the Blue River, and the Dakota Sandstone of Early Cretaceous age crops out principally along the west side of the river. Downstream from Dillon Reservoir, alluvial deposits occur along the main channel of the Blue River. 16