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cc w ac 0 a 00 0 0Z U N C0 d0 wZ_ H z oa Zm 0 g D IL O n 400 350 300 250 200 150 100 50 EXPLANATION ¦ Arizona ® Colorado New Mexico ® Utah F]Wyoming R 1930 1940 1950 1960 1970 1980 Figure 4. Changes in population between 1930 and 1980 in Upper Colorado River Basin (data from the U.S. Bureau of Cen- sus, 1982). systems of canals and ditches. Consumptive use by crops averaged 1.8 million acre-ft/yr during the 1973-82 water years, about 13 percent of the annual virgin streamflow of the Colorado River at Lees Ferry, Ariz., during the same period (U.S. Bureau of Reclamation, written commun., 1985). Virgin streamflow is the streamflow that would have occurred in the absence of human intervention upstream. It is an estimate of all water consumptively used or exported from the basin. Irrigated agriculture is the largest source of dissolved solids related to human activities in the Upper Colorado River Basin. Irrigation-return flows generally have a higher dissolved-solids concentration than the applied water, because of the loading effect of salt dissolution in the soil-aquifer system and the concentrating effect of evapotranspiration. Many areas in the basin did not contribute substantially to runoff, and thus to dissolved-solids loading, until the advent of irrigation. Because irrigation practices began before streamflow and water-quality records were initiated, the in- crease in dissolved-solids loading in the basin can only be estimated. Transbasin Exports and Imports Although much of the 'Upper Colorado River Basin is arid, large volumes of water are exported to other basins. Diversions began in 1892 when the Grand River Ditch was constructed in the headwaters of the Colorado River near Grand Lake, Colo. Total exports averaged less than 2,000 acre-ft/yr at the turn of the century. Exports increased to ap- proximately 100,000 acre-ft by 1920, 200,000 acre-ft by 1940, and 500,000 acre-ft by 1955 (fig. 6). During the 1973-82 water years, annual transbasin exports averaged 739,000 acre-ft, about 5 percent of the virgin streamflow? of the Colorado River at Lees Ferry, Ariz. For the same period, 54 percent of the total exports went to the Platte River basin, 17 percent went to the Arkansas River basin, 15 per- cent went to the Rio Grande basin, and 14 percent went to the Great Basin in Utah. Diversions that export water from the Upper Colorado River Basin are listed in table 1. Transbasin exports from the Upper Colorado River Basin tend to increase the downstream concentration of dissolved solids. Exports primarily are from the headwater regions, where dissolved-solids concentrations are low. The removal of this relatively pure water from the headwater regions leaves less water for dilution downstream. Historically, only one transbasin diversion has brought water into the Upper Colorado River Basin. The Tropic and East Fork Canal has diverted water from the Great Basin into the Paria River basin since 1892, averaging 4,800 acre-ft annually during 1974-83 (Upper Colorado River Commis- sion, 1984). Reservoirs The first artificial reservoirs in the Upper Colorado River Basin were constructed during the 1890's, primarily for water storage and irrigation. Strawberry Reservoir, com- pleted in 1912, was the first reservoir that had a capacity greater than 100,000 acre-ft; it stored water for delivery through Strawberry Tunnel to the Great Basin in Utah. Reser- voirs were important for settlement as well as for agriculture because they increased the year-round availability of water in a region characterized by general aridity and seasonal streamflow resulting from snowmelt. Completion of the Colorado River Storage Project during the the 1960's in- creased the combined capacity of reservoirs in the basin to about 38 million acre-ft, more than three times the mean annual flow measured at Lees Ferry, Ariz. Lake Powell, formed by Glen Canyon Dam, is by far the largest reser- voir and has a total capacity of 27 million acre-ft. It is about the size of Lake Mead, which is formed by Hoover Dam in the Lower Colorado River Basin. Eighty-two reservoirs in the Upper Colorado River Basin have a normal capacity greater than 5,000 acre-ft (table 2). The major effects of reservoirs on streamflow are associated with the regulation of streamflow and evapora- tion losses from the water surface. Reservoir regulation tends to decrease the seasonal variability in streamflow; it increases the low-flow volumes during late summer, autumn, and winter and decreases the peak flows occurring during the snowmelt season, April through June. However, discharge patterns downstream from reservoirs become more complex when the timing and magnitude of releases are controlled by power-generation requirements or by diversion projects. Evaporation from Lake Powell during 1973-82 averaged 8 Characteristics and Trends of Streamflow and Dissolved Solids in the Upper Colorado River Basin