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contain large quantities of soluble material; streamflow con- tains relatively high concentrations of dissolved magnesium and bicarbonate but low concentrations of dissolved sulfate. During the snowmelt season, calcium and bicarbonate are the predominant ions. Sodium, calcium, magnesium, and bicarbonate predominate during the low-flow season. No statistically significant annual monotonic trends were detected during 1969-79. Gunnison Subregion The drainage area of the Gunnison subregion (pl. 1) is about 8,000 mil. The Gunnison River is the largest tributary to the Colorado River in Colorado and originates in high mountainous terrain. Irrigation in the Gunnison subregion began about 1890, and more than 200,000 acres was irrigated during 1986, of which about 90,000 acres is along the Uncompahgre River, about 20,000 acres is along Tomichi Creek in the upper Gunnison area, and most of the remainder is near the town of Delta, Colo., in the lower reaches of the Gunnison basin. The large Uncompahgre and lower Gunnison areas are underlain by Mancos Shale. Irri- gation water applied to land underlain by Mancos Shale causes weathering and dissolution of mineral salts from the soils and underlying shale. As a result, the water returns to the river with a larger dissolved-solids load than it had before it was diverted (Iorns and others, 1965). Annual agricultural dissolved-solids loading has been estimated at 360,000 tons from the Uncompahgre Valley and 480,000 tons from the lower Gunnison area (U.S. Department of the Interior, 1985), which makes the Gunnison subregion the largest agricultural source of dissolved solids in the Upper Colorado River Basin. Three small transbasin diversions, the Larkspur, Tabor, and Tarbell ditches export water from the Gunnison River subregion to the Arkansas River and Rio Grande basins. Large volumes of water are diverted within the subregion through the Gunnison Tunnel, which transports water from the Gunnison River to the Uncompahgre Valley. Diversions through the tunnel began in 1910, totaled 117,000 acre-ft during 1915, 250,000 acre-ft during 1930, and 352,000 acre- ft during 1950. Taylor Park Dam was built in 1935 for storage as part of the Uncompahgre Project. Three large dams were built for power generation and water storage on the main stem of the Gunnison River as part of the Colorado River Storage Project. Together the reservoirs constitute the Wayne N. Aspinall Unit, formerly known as the Curecanti Unit. The largest and farthest upstream is Blue Mesa Reservoir; the others are Morrow Point Reservoir and Crystal Reservoir. Uncompahgre River at Delta, Colo. (site 12) Streamflow at site 12 (table 3, pl. 1) is affected by im- ports from the Gunnison Tunnel from April through October and by substantial evapotranspiration from irrigated croplands. The streamflow hydrograph for site 12 is char- acterized by a modest snowmelt-runoff peak during May and June and a prominent irrigation-return flow period lasting from September through mid-November (fig. 10A). Calcium and sulfate are the predominant ions during the entire year. Dissolved sulfate makes up about 60 percent of the dissolved- solids load. The river also transports large loads of dissolved sodium, magnesium, and bicarbonate, but only a small load of dissolved chloride. Annual flow-weighted dissolved-solids concentration averages about 1,260 mg/L (table 5). Annual flow of the Uncompahgre River at site 12 did not change significantly during 1959-80, but dissolved-solids concentration and load have decreased since the late 1960's. Annual monotonic-trend analyses indicated highly signifi- cant decreases in median annual values of dissolved-solids concentration of 20.4 mg/L per year and flow-adjusted con- centration of 19.3 mg/L per year, and a significant decrease in median annual dissolved-solids load of 3,810 tons/yr (table 6). During the 22-year period, these trends represent a 30-percent decrease in median annual concentration, a 28-percent decrease in median annual flow-adjusted concen- tration, and a 22-percent decrease in median annual load. All major constituents, except dissolved bicarbonate, de- creased significantly in concentration and flow-adjusted concentration. Flow-adjusted concentration decreased dur- ing all months. The reason for this decline is unknown, but it may be related to regulation and storage by Blue Mesa Reservoir, which is downstream from the irrigated areas in the upper reaches of the Gunnison River and upstream from the diversion point of the Gunnison Tunnel. Gunnison River near Grand )unction, Colo. (site 13) Streamflow hydrographs for site 13 (table 3, pl. 1) are shown in figure 10B. During 1897-1906, a flattening of the snowmelt-runoff peak from diversions for agriculture occurred; the peak further decreased after 1906 because of irrigation diversions through the Gunnison Tunnel. Regula- tion by reservoirs in the Wayne N. Aspinall Unit after 1965 has not decreased the annual flow of the Gunnison River but has approximately halved the flow during the snowmelt season and doubled the flow during the base-flow period. Streamflow and dissolved-solids concentration are less variable throughout the year, and from year to year. As at site 12, the dissolved-solids composition is predominantly calcium and sulfate, with small quantities of sodium, magnesium, and bicarbonate. Although the chemical com- position of streamflow is similar to that of the Uncompahgre River, the dissolved-solids concentration is lower because of dilution from the main stem of the Gunnison River. The period of record at site 13 was divided into a preintervention period (1934-65) and a postintervention period (1966-83), based on the completion of Blue Mesa Dam. The annual step trends in streamflow and dissolved- solids concentration were not significant, but a marginally significant decrease of 119,000 tons in dissolved-solids load Gunnison Subregion 27