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released from the reservoir into the Strawberry River. As part of the Central Utah Project, the reservoir was enlarged in 1973 to a capacity of 1.1 million acre-ft (table 2). However, the remainder of the project has not been com- pleted, and the reservoir had not been filled by 1983. Vermillion Creek near Hiawatha, Colo. (site 35) Vermillion Creek drains an arid, geologically diverse area before emptying into the Green River in the northwest- ern corner of Colorado. Site 35 (table 3, pl. 1) has about one-third the streamflow of Vermillion Creek near its mouth, but the dissolved-solids concentrations are similar. Stream- flow at site 35 is characterized by a small snowmelt-runoff peak, occasional flash floods, and minimal or no flow dur- ing the remainder of the year. Mean annual flow-weighted dissolved-solids concentration averages 921 mg/L (table 7). Sodium and sulfate are the predominant ions. Concentration and chemical composition do not vary greatly throughout the year. Yampa River below diversion, near Hayden, Colo. (site 36) Streamflow at site 36 (table 3, pl. 1) averages about 700,000 acre-ft/yr (table 7) and primarily is composed of snowmelt runoff that has minimal dissolved solids. The mean annual flow-weighted dissolved-solids concentration is 90 mg/L. Calcium and bicarbonate are the predominant ions throughout the year. Water from the Elk River, which drains the virtually insoluble Precambrian rocks of the Park Range, dilutes the dissolved-solids concentration of the Yampa River upstream from Hayden, Colo. Wilson Creek near Axial, Colo. (site 37) Wilson Creek is a tributary to Milk Creek and drains a small area underlain principally by the Williams Fork For- mation. The streamflow hydrograph for site 37 (table 3, pl. 1) is characterized by a sharp snowmelt-runoff peak in mid- May that is unaffected by reservoirs or diversions (fig. 18A). Although the total runoff from the drainage basin is small, base flow is rather constant. The mean annual flow-weighted dissolved-solids concentration is 787 mg/L (table 7), pre- dominantly as magnesium and sulfate. The proportions of dissolved sodium and chloride are larger than in the main- stem waters of the Yampa River at site 36. During the high- flow season, the concentrations of magnesium, calcium, sodium, bicarbonate, and sulfate are approximately equal. The proportions of sodium, sulfate, and, notably, magnesium increase during the low-flow season. Brine disposal from oil- drilling operations in the upper part of the drainage basin may have contributed substantial quantities of dissolved solids to the stream (Turk and Parker, 1982). Yampa River near Maybell, Colo. (site 38) The mean annual streamflow of the Yampa River at site 38 (table 3, pl. 1) exceeds 1 million acre-ft/yr (table 7). A large snowmelt-runoff peak occurs during May and June, followed by a long period of steady base flow (fig. 18B). Compared to the Yampa River at site 36, streamflow is 54 percent larger and mean annual dissolved-solids load is 162 percent larger, the increase principally being dissolved sulfate, bicarbonate, and calcium. The mean annual flow- weighted dissolved-solids concentration is 152 mg/L (table 7). Calcium and bicarbonate are the predominant ions dur- ing the high-flow season, and calcium, sodium, and bicar- bonate predominate during the low-flow season. For 1951-83, annual monotonic-trend analyses indi- cated highly significant increases in median annual dissolved- solids concentration of 0.9 mg/L per year and in median annual flow-adjusted concentration of 1.2 mg/L per year. Trends indicated a significant increase in median annual dissolved-solids load of 3,380 tons/yr (table 8). These in- creases represent a 21-percent change in median annual dissolved-solids concentration, a 29-percent change in flow- adjusted concentration, and a 65-percent change in dissolved- solids load during the 33-year period of record. Highly significant increasing trends were determined for annual flow-adjusted concentrations of magnesium, sodium, and sulfate. Flow-adjusted concentration increased significantly during all months except April and August. Iorns and others (1965) reported that although contribu- tions from rocks of Cretaceous and Tertiary age in the Yampa River basin had large proportions of dissolved magnesium, sodium, and sulfate, the dissolved-solids concentration of the river increased only moderately because the volume of runoff was small. These same constituents have increased flow- adjusted concentrations for site 38. The major land-use change in the Yampa River basin has been the expansion of coal-resource development since the early 1960's (Steele and others, 1979). Both gypsum, a sulfate mineral, and pyrite, which produces sulfate during oxidation, occur in the spoils of surface-mined coal. Turk and Parker (1982) reported that a principal effect of coal mining may be an increase ip the relative concentration of dissolved sulfate in surface water. Increases in dissolved-solids and dissolved-sulfate concen- trations at site, 38 are moderately small, but they are signifi- cant. The proportion of dissolved sulfate increased from about 24 to 36 percent during 1951-83. The increase may have been caused by leaching of soluble minerals from spoils and disturbed lands. McWhorter and others (1975) reported that the dissolved-solids load from spoils at the Edna Mine in Colorado was 10 times greater than that produced upstream along Trout Creek. Runoff from spoils in the Yampa River basin produces three to six times the runoff from undisturbed land, and springs have been observed at the toe of spoils, changing intermittent streams into perennial streams. A typical spoils pile may yield annually 3 to 5 inches of runoff having a dissolved-solids concentration of 1,500 to 2,500 mg/L (R.S. Williams, U.S. Geological Survey, oral com- mun., 1985). Warner (in Steele and Hillier, 1981) reported that leachate from spoils could produce a slow spread of degraded ground water throughout a period of decades. Middle Green Subregion 41