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Much study has been done in the drainages of Piceance and Yellow Creeks to evaluate potential effects from the possible development of oil-shale deposits. The Uinta For- mation is exposed throughout most of the area. The Parachute Creek Member of the Green River Formation is the prin- cipal bedrock aquifer in the area and is exposed along streams. Water in the Parachute Creek Member has high con- centrations of dissolved sodium, bicarbonate, and chloride (Weeks and others, 1974). An area of about 5,000 acres is irrigated in the Piceance Creek and Yellow Creek basins, with about 4,000 acre-ft of water depleted from mid-March through November. White River below Meeker, Colo. (site 44) The streamflow hydrograph for site 44 (table 3, pl. 1) is characterized by substantial base flow and a snowmelt- runoff peak that is partially depleted by irrigation (fig. 19A). Ionic composition is predominantly calcium and bicarbonate during the high-flow season and calcium, sulfate, and bicar- bonate during the low-flow season. Sulfate is the predomi- nant component of dissolved-solids load because of the upstream irrigation on areas underlain by Mancos Shale and because of pyrite oxidation in the Green River Formation. The mean annual flow-weighted dissolved-solids concen- tration is 284 mg/L (table 7). During 1974-83, after the plugging of the Meeker well (1968), dissolved chloride averaged about 11,000 tons/yr out of a total dissolved-solids load of 178,000 tons/yr (table 7). The period after the plug- ging of the other three wells (1981-83) was too short for an intervention analysis. However, the proportions of sodium and chloride at site 44 decreased substantially during 1982 and 1983. Annual monotonic-trend analyses indicated a highly significant decrease in median annual flow-adjusted concen- tration for the entire period of record. This decrease was 3.5 mg/L per year, representing a 12-percent change in the median annual concentration during the 10 years of record. Decreases in the flow-adjusted concentrations of calcium, sodium, and bicarbonate contributed to this trend. Piceance Creek below Ryan Gulch near Rio Blanco, Colo. (site 45) Site 45 is about 12 mi upstream from site 46 (table 3, pl. 1). The streamflow hydrographs for sites 45 and 46 on Piceance Creek show similar distributions; streamflow at the mouth (site 46) averaged 5 ft3/s more than at site 45 up- stream (fig. 19B). Agricultural depletions cause decreased streamflow during the irrigation season, notably during April; ground-water return flows make up most of the streamflow from November through February. Heavy snowfalls occa- sionally increase streamflow during February and March, and high-intensity thunderstorms occasionally increase streamflow during late summer (Weeks and others, 1974). No statistically significant annual monotonic trends were detected at this site. Piceance Creek at White River, Colo. (site 46) Mean annual flow-weighted dissolved-solids concen- trations in Piceance Creek increase from 926 mg/L at site 45 to 1,240 mg/L at site 46 (table 7). Eighty-eight percent of this increase is a result of increases in sodium and bicar- bonate concentrations. The relative proportions of sodium, bicarbonate, and chloride increase between the two sites. Sodium and bicarbonate are the predominant ions throughout the year at both sites. The proportion of calcium is small, and the equivalent concentration of magnesium exceeds that of calcium. The relatively high concentrations and the pre- dominance of sodium and bicarbonate apparently result from subsurface flow through the Parachute Creek Member of the Green River Formation, which is connected to the surface by fracturing and is exposed in the lower reaches of the Piceance Creek basin. Annual monotonic-trend analyses for 1971-83 indicated a highly significant decrease in median annual dissolved- solids concentration of 44.1 mg/L per year and a marginal- ly significant decrease in median annual flow-adjusted concentration of 24.2 mg/L per year (table 8). During the 13 years of record, these decreases represent a 34-percent change in median annual concentration and a 20-percent change in median annual flow-adjusted concentration. Both trends may be caused by changes in irrigation practice associated with oil-shale leasing. Yellow Creek near White River, Colo. (site 47) Streamflow at site 47 (table 3, pl. 1) is generally less than 4 ft3/s (fig. 19C). Occasional winter snowmelt events and summer thunderstorms generate flood peaks as large as 500 ft3/s. Snowmelt runoff that occurs during April and May is diverted for irrigation. During most of the year, the flow- weighted dissolved-solids concentration is fairly constant, averaging 2,140 mg/L (table 7). Sodium and bicarbonate are the predominant ions throughout the year and account for 66 percent of the mean annual dissolved-solids load. Some irrigation-return flow is indicated during October and November. Chemical composition is very similar to that of Piceance Creek, except that little snowmelt is available to dilute ground-water inflow during the spring and early sum- mer. The proportion of dissolved calcium is extremely small, only 1.4 percent of the total load of dissolved solids. White River above Rangely, Colo. (site 48) Streamflow at site 48 (table 3, pl. 1) has approximate- ly the same volume and seasonal pattern as at site 44. Con- centrations of sodium, bicarbonate, and sulfate are higher, and the mean annual flow-weighted dissolved-solids concen- tration averages 391 mg/L (table 7). The increased dissolved sodium and bicarbonate are contributed by Piceance and Yellow Creeks and by water discharged from the Parachute Creek Member of the Green River Formation. The increased 44 Characteristics and Trends of Streamflow and Dissolved Solids in the Colorado River Basin