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San Rafael River. Site 55 (table 3, pl. 1) is downstream from Joes Valley Reservoir but upstream from diversions to the agricultural area of the San Rafael River basin. Construc- tion of Joes Valley Reservoir preceded the beginning of water-quality sampling at this site. Streamflow of Cotton- wood Creek at site 55 has been controlled almost complete- ly by reservoir releases since 1966. Snowmelt runoff is stored in Joes Valley Reservoir during May, then released through- out the irrigation season; there is a sharp decline in stream- flow at the end of October (fig. 20C). Because this site is upstream from the agricultural areas, mean annual flow- weighted dissolved-solids concentration is relatively low (225 mg/L; table 7), and it is almost constant throughout the year. Calcium and bicarbonate are predominant ions throughout the year; magnesium also becomes predominant during the low-flow season. The proportions of dissolved sodium and sulfate are small. San Rafael River at San Rafael Bridge Campground, near Castle Dale, Utah (site 56) Site 56 (table 3, pl. 1) is about 13 mi downstream from the confluence of Huntington, Cottonwood, and Ferron Creeks. It is downstream from almost all the agricultural land in the San Rafael River basin. Substantial quantities of dissolved solids from irrigation-return flows and from springs and seeps in the Curtis and Carmel Formations also enter the river upstream from this site. Mean annual streamflow is only slightly greater than at site 55, but dissolved-solids load increases almost eightfold. Dissolved sodium and sulfate compose 72 percent of the dissolved-solids load. Sodium and sulfate are the predominant ions throughout the year. Calcium and magnesium are approximately equal to sodium during the high-flow month of June. Streamflow during May re- mains relatively low because of irrigation diversions. Although the proportions of dissolved sodium and sulfate are much larger than at site 55, the available data do not indicate the relative magnitude of dissolved-solids load contributed by the irrigated Mancos Shale area versus natural dissolved- solids load from the San Rafael Swell area. However, based on the minimal runoff in the San Rafael Swell area, most of the dissolved-solids load may be leached from the Mancos Shale benches. Agricultural soils in the area have deteriorated from salt accumulation, and 4,600 acres was eliminated from agricultural use during 1976. San Rafael River near Green River, Utah (site 57) Site 57 (table 3, pl. 1) is about 20 mi upstream from the confluence with the Green River, in an area referred to as the San Rafael Desert. Comparison of data from this site and site 56 indicates that streamflow and dissolved-solids load increase slightly, but dissolved-solids concentration is about the same. The proportion of dissolved sulfate increases in the reach between the two sites. Since the filling of Joes Valley Reservoir, the snowmelt-runoff peak occurs later in the season; otherwise, streamflow has not changed greatly (fig. 20D). The period of record was divided into a preinterven- tion (1947-65) and a postintervention (1966-83) period based on the filling of Joes Valley Reservoir. No statistically signifi- cant annual step trends or annual monotonic trends were detected in the data. Monthly step trends indicated a redis- tribution of streamflow, probably because of regulation by the reservoir. Streamflow decreased significantly during January, February, and May and increased significantly dur- ing July and October. General Trends in the Green Region The filling of Flaming Gorge Reservoir and, to some extent, Fontenelle Reservoir has decreased seasonal variabil- ity in streamflow along the main stem of the Green River. Streamflow at site 34 near Greendale, Utah, is virtually con- stant throughout most years. Significant increases in annual dissolved-solids concentration and load at site 34 resulted from dissolution of mineral salts in bed and bank material inundated by Fontenelle and Flaming Gorge Reservoirs, from irrigation-retum flows in the Big Sandy River basin, and from increases in streamflow. The Eden Valley Project, begun dur- ing the 1950's, increased the dissolved-solids load of the Big Sandy River because of large irrigation-return flows. The Yampa River contributes almost the same volume of water as does the Green River upstream from their con- fluence. Because the dissolved-solids concentration in the Yampa River is lower than in the Green River, the Yampa River inflow has a diluting effect. Increases in dissolved- solids load and concentration in the Yampa River may be related to surface mining of coal in the basin. The White River contributes a large load of dissolved solids derived from natural and anthropogenic sources. Re- cent efforts to control the flow of saline ground water from several abandoned wells near Meeker, Colo., have produc- ed a significant decrease in dissolved-solids concentration downstream. The Price and San Rafael Rivers also transport large quantities of dissolved solids from agricultural sources. Streamflow has high concentrations of dissolved sodium and sulfate, resulting from irrigation-return flow from agricultural areas underlain by Mancos Shale and from contributions from the Carmel Formation. Reductions in irrigated area may have caused decreases in dissolved-solids concentration in the Price River. At site 54 near Green River, Utah, near the downstream end of the Green region, step-trend analysis indicated decreasing seasonal variability. Dissolved-solids concentra- tion increased by 29 mg/L because of changes upstream from Flaming Gorge Dam. Decreased concentration of dissolved chloride was caused by the decrease in dissolved-solids input from the Meeker Dome area. 48 Characteristics and Trends of Streamflow and Dissolved Solids in the Colorado River Basin