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Main-Stem Subregion The main-stem subregion consists of the remainder of the San Juan region including the Dirty Devil and Paria Rivers and the main stem of the Colorado River. An area of about 19,000 acres is irrigated in the Dirty Devil River basin, which empties into the upper end of Lake Powell, upstream from the San Juan River. The Fremont River and Muddy Creek merge near Hanksville, Utah, to form the Dirty Devil River. Two small reservoirs, Johnson and Mill Meadow, store water for irrigation-season releases. The Carmel Formation and Mancos Shale are major contributors of dissolved solids to the stream system. As in the San Rafael and Price River basins to the north, large tracts of irrigated land are underlain by Mancos Shale. Only about 3,300 acres is irrigated in the Paria River basin, which empties into the Colorado River downstream from Glen Canyon Dam. The Paria River basin receives the only transbasin import into the entire Upper Colorado River Basin. The Tropic and East Fork Canal has imported water from the Sevier River in the Great Basin since 1892. An- nual imports averaged 4,840 acre-ft during 1974-83. The imports were used to irrigate land underlain, by Tropic Shale. Lake Powell is formed by Glen Canyon Dam, 16 mi upstream from Lees Ferry, Ariz. The reservoir extends more than 180 mi upstream and inundates more than 250 mil of canyon and desert. Total storage capacity of the reservoir is about 27 million acre-ft. Evaporation from the lake sur- face averaged 517,000 acre-ft/yr during 1973-82 (U.S. Bureau of Reclamation, 1982; J. Osterberg, U.S. Bureau of Reclamation, oral commun., 1986). The 710-ft-high dam was begun in 1957 and completed in 1963. The reservoir serves multiple purposes; it is a major recreation area and a source of hydroelectric power. It provides flood control on the Colorado River and also is a water supply for meeting the terms of the Colorado River Compact during extended droughts. Storage began in Lake Powell on March 13, 1963, but the severe drought during 1963 and 1964 slowed the fill- ing. The minimum power-pool elevation of 3,490 ft was reached in August 1964, and the Glen Canyon powerplant was placed in operation that September. Excess spring runoff during 1965 was passed on to Lake Mead, producing a max- imum streamflow of 56,600 ft3/s. The initial filling period ended July 31, 1965, when a normal streamflow release pro- gram was begun (fig. 24). At the end of water year 1965, Lake Powell was 31 percent full. Annual releases of 8 to 10 million acre-ft were maintained during water years 1966-79. Filling continued until June 22, 1980, when total storage reached 27 million acre-ft at a lake-surface eleva- tion of 3,700 ft. This marked the end of the filling period, and since then, the reservoir has remained relatively full. Exceptionally large inflows during 1983 required releases over the dam's spillway, which provided a maximum streamflow of 92,600 ft3/s. Releases during 1984 remained consistently between 25,000 and 30,000 ft3/s, except for a period during June and July, when more than 2 million acre-ft was released to prevent overtopping of the dam. During the drought years of 1977 and 1981, releases from the reservoir exceeded inflow. Dirty Devil River above Poison Spring Wash, near Hanksville, Utah (site 58) Site 58 (table 3, pl. 1) is less than 20 mi upstream from the mouth of the Dirty Devil River. Although the headwaters are in the Wasatch Plateau, most of the basin is arid. Runoff is minimal, and streamflow is affected by agricultural diver- sions. Spring runoff is greatly depleted, and median flow in June and July is almost zero because of continued irriga- tion (fig. 25A). Intense storms and winter runoff supply most of the annual streamflow; spring runoff only occasionally exceeds depletions for irrigation. Most of the dissolved-solids load results from the dissolution of gypsum and halite and enters the stream from diffuse ground-water inflow. Calcium and sulfate are the predominant ions throughout the year. Sulfate is 51 percent of the dissolved-solids load, and chloride is 10 percent of the dissolved-solids load, an unusually large percentage. Mean annual flow-weighted dissolved-solids con- centration was 1,110 mg/L during 1969-76 (table 9). Colorado River at Hite, Utah (site 59) Site 59 (table 3, pl. 1), upstream from the mouth of the San Juan River, has been inundated by the waters of Lake Powell. Water-quality data exist from 1951 through 1956 and seem to represent the preintervention period. Mean annual flow-weighted dissolved-solids concentration for the period was 580 mg/L (table 9). The dissolved-solids con- centration in streamflow downstream from Hite, Utah, is diluted by the inflow of the San Juan River. Seasonal stream- flow at site 59 indicated a well-defined snowmelt-runoff peak during May and June and an average base flow of about 5,000 ft3/s throughout the winter months. Calcium, bicarbonate, and sulfate are the predominant ions during the high-flow season. Sodium and sulfate are the predominant ions during the low-flow season. Sulfate averages 42 percent of the dissolved-solids load throughout the year. The monthly load of chloride had much less variation than that of the other constituents, which indicates a relatively constant supply rate. Colorado River at Lees Ferry, Ariz. (site 69) The streamflow hydrograph for site 69 (table 3, pl. 1), prior to 1962, was characterized by a snowmelt-runoff season peakin g May or early June at more than 50,000 ft3/s, and a well-sustained base flow of 5,000 ft3/s or more (fig. 25B). Daily streamflow during 1922-62 ranged from 1,000 to 124,000 ft3/s. Mean monthly dissolved-solids con- centration during 1942-62 varied from 300 mg/L during June to 1,000 mg/L during January. Calcium and bicarbonate were the predominant ions during the high-flow season (May and June). Sodium, calcium, and sulfate were predominant dur- ing the remainder of the year. Sulfate was 41 percent of the Main-Stem Subregion 55