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flow during September through February. Median annual dissolved-solids load significantly increased by 1,740 tons/yr, and median annual flow-adjusted concentration significant- ly increased by 13.9 mg/L per year. These increases repre- sent a 79-percent change in median annual streamflow, a 53-percent change in median annual load, and a 17-percent change in median annual flow-adjusted concentration dur- ing the period of record. Based on the monthly time series of streamflow and dissolved-solids load, irrigation-return flows increased until about 1966 and since then have re- mained relatively stable. Big Sandy River at Gasson Bridge, near Eden, Wyo. (site 28) Site 28 (table 3, pl. 1) is about 10 mi downstream from site 27. Streamflow is similar at the two sites but averages 21 ft3/s greater at site 28 (fig. 15D) because of irrigation- return flows that enter the stream by seepage through shallow aquifers. Mean annual dissolved-solids load increased from 82,000 tons at site 27 to 163,000 tons at site 28 (table 7). Dissolved sodium and sulfate composed 82 percent of the increased dissolved-solids load. The mean annual dissolved- sulfate load of 100,000 tons almost equaled the dissolved- sulfate load of the Green River at site 25, with a drainage area of 4,280 mil, compared to only 1,720 mil at site 28. Bitter Creek above Salt Wells Creek, near Salt Wells, Wyo. (site 29) The headwaters of Bitter Creek are not in a moun- tainous region but are in a semiarid area along the southwest border of the Great Divide Basin. Consequently, streamflow of Bitter Creek at site 29 (table 3, pl. 1) has only a modest, early snowmelt-runoff season (fig. 15E). The stream dries up during the summer, except after thunderstorms, which produce short, intense flood peaks. Thunderstorms decrease as the weather cools during September, and the creek general- ly is dry through the winter months. The flow-weighted dissolved-solids concentration averaged about 1,300 mg/L during 1977-81 (table 7). Chemical composition, as in the Big Sandy River, is predominantly sodium and sulfate dur- ing all seasons. The proportions of dissolved sodium and chloride are larger and the proportions of dissolved calcium and sulfate are smaller than in Big Sandy River. There is no significant water use upstream from site 29. Downstream from the site, Salt Wells Creek converges with Bitter Creek, increasing streamflow and dissolved-solids concentration before the creek empties into the Green River. Lowham and others (1982) described the water chemistry of Salt Wells Creek in detail. They reported that the dissolved-solids con- centration ranges from 100 mg/L in the headwaters to 3,000 mg/L near the mouth. Green River near Green River, Wyo. (site 30) Site 30 (table 3, pl. 1) is about 50 mi downstream from Fontenelle Dam, 0.1 mi downstream from Bitter Creek, and 4 mi upstream from the high-water line of Flaming Gorge Reservoir. Samples for water-quality analyses are collected upstream from the streamflow-gaging station and Bitter Creek. The seasonal pattern of streamflow has been altered slightly by regulation of flow at Fontenelle Reservoir (fig. 15F). The predominant ions are calcium and bicarbonate dur- ing the snowmelt-runoff season (May through August). Calcium, sodium, and sulfate predominate during the re- mainder of the year. The flow-weighted dissolved-solids con- centration averaged 309 mg/L during 1964-83 (table 7). The site was evaluated for annual step trends caused by Fontenelle Dam. The period of record was separated into preintervention (1952-63) and postintervention (1964-83) periods. The postintervention period generally has had larger streamflow, but because of the variability from year to year, the annual step trend in streamflow was not significant. The step-trend analysis indicated a significant increase in annual dissolved-solids load of 118,000 tons, a 26-percent change from the preintervention median load (table 4). Most of this increase was attributable to changes in sodium and sulfate loads, possibly from dissolution of minerals from the bank material of Fontenelle Reservoir and from increased dissolved-solids loads from the Big Sandy River. Monthly step trends indicated that streamflow and dissolved-solids load increased during the low-flow season because of releases from Fontenelle Reservoir (fig. 16). However, they did not decrease during the high-flow season because of the generally_ larger flows during the postintervention period, especially during 1983. For the preintervention period, annual monotonic-trend analyses indicated a marginally significant decrease in dissolved-solids load of 15,600 tons/yr. This trend represents a 34-percent decrease in the median annual load during the 12-year period. This trend may be related to climate; several years of high streamflow occurred early in the period. No statistically significant annual monotonic trends were detected for the postintervention period. Blacks Fork near Lyman, Wyo. (site 31) Blacks Fork, upstream from site 31 (table 3, pl. 1), drains the northern slopes of the Uinta Mountains (fig. 2). Despite large tracts of irrigated land, flow regulation was not begun until late 1981 (Meeks Cabin Dam), and the streamflow hydrograph indicates a well-defined snowmelt- runoff peak during June (fig. 15G). The flow-weighted dissolved-solids concentration averaged 697 mg/L during 1963-83 (table 7). The predominant ions are sodium and sulfate throughout the year. Dissolved sodium and sulfate compose 70 percent of the dissolved-solids load during the low-flow season. This chemical composition is affected by return flow from irrigated land underlain by Cretaceous and Tertiary shale. No statistically significant trends were in- dicated for the period of record. Blacks Fork near Little America, Wyo. (site 32) The streamflow at site 32 (table 3, pl. 1) includes the streamflow at site 31 plus the streamflow of Hams Fork and Upper Green Subregion 37