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SITE 21 Colorado River near Cisco, Utah (site 21) Site 21 (table 3, pl. 1) is the farthest downstream site in the Grand region. The streamflow hydrograph for this site depicts the progressive depletion and regulation of flow in the Grand region since streamflow-gaging records began in 1914 (fig. 12F). Since 1966, mean annual streamflow has averaged about 4.8 million acre-ft and mean annual dissolved-solids load has averaged about 3.7 million tons (table 5). Chemical composition is affected by irrigation- return flows from Mancos Shale areas, such as the Grand Valley, and inflow from the saline springs in the Dotsero- Glenwood Springs area and the Dolores River basin. Dur- ing snowmelt runoff, calcium, sulfate, and bicarbonate are the predominant ions; during base flow, sodium and sulfate predominate. Because of inflow from the Dolores River, the proportion of chloride is larger at site 19 than at site 18 upstream. The period of record was separated into three inter- vals: A preintervention period (1929-49), a middle period (1950-65), and a postintervention period (1966-83). The intervals were divided based on construction of the Alva B. Adams Tunnel/Lake Granby (1950, when full operation of the project began) and Blue Mesa Reservoir (1966). Annual step trends resulting from Alva B. Adams Tunnel/Lake Granby between 1929-49 and 1950-65 indicated a signifi- cant decrease in dissolved-solids load of 594,000 tons/yr (table 4). This decrease was 14 percent of the preinterven- tion median. The loads of dissolved sulfate, bicarbonate, and sodium decreased during the snowmelt season, and the dissolved chloride load decreased during the low-flow season. Annual step-trend analysis, for changes resulting from Blue Mesa Reservoir between the periods 1950-65 and 1966-83, detected no significant changes in annual medians, except for a decrease in dissolved-sulfate load of 137,000 tons/yr. Monthly step trends indicated that from September through March, streamflow increased and dissolved-solids concen- tration decreased, probably due to increased reservoir regula- tion (fig. 13). Annual monotonic trends were analyzed for the three intervals at site 21. For the preintervention period, increased streamflow and decreased dissolved-solids concentration were detected during the low-flow season. These trends cor- responded to trends detected at sites 5 and 10 and probably are from winter releases from Green Mountain and Taylor Reservoirs. No statistically significant annual trends were detected for the preintervention and middle periods. For the postintervention period, a marginally significant decrease in median annual flow-adjusted concentration of 2.9 mg/L per year was detected (table 6). This trend represents a 9-percent decrease in the median annual flow-adjusted concentration during the 18-year period (1966-83) and corresponded to similar decreases in the Gunnison River subregion. 100,000 w JO LL 50,000 LL W Cr ?¢ U ? ¢ o ?Z _50,000 (n 15,000 o(n M Z 0 ° 0 0Z w- 0 Q 15,000 Lno 0 -30,000 MONTH EXPLANATION Highly significant (p 50.01) ® Significant (0.01< p<_ 0.05) ® Marginally significant (0.05<p5_0.10) Figure 13. Step trends at site 21 (Colorado River near Cisco, Utah) from 1950-65 to 1966-83. General Trends in the Grand Region Annual streamflow in the Grand region has been progressively depleted by transbasin exports. Exports and reservoir regulation have decreased streamflow during the snowmelt-runoff season, and slightly increased streamflow during the winter along the main stems of the Colorado and Gunnison Rivers. Annual step-trend analysis indicated decreases in streamflow in the Grand region. The Alva B. Adams Tunnel/ Lake Granby export, which is the largest depletion in the entire Upper Colorado River Basin, was the probable cause of changes in streamflow and dissolved-solids concentration at site 5 (Colorado River near Glenwood Springs, Colo.) and changes in streamflow and dissolved-solids load at site 10 (Colorado River near Cameo, Colo.). The filling of Blue Mesa Reservoir decreased the monthly variability of stream- flow, dissolved-solids concentration, and dissolved-solids load in the Gunnison River and may have contributed to decreasing annual monotonic trends in concentration and load. Significant decreases in dissolved-solids concentration and load also were detected in the Eagle River basin. Dissolved-solids load, mainly dissolved sodium and chloride, increased in the Dolores River basin. Annual step trends at the most downstream site in the Grand Region (site 21, Colo- 32 Characteristics and Trends of Streamflow and Dissolved Solids in the Colorado River Basin