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900 <br />800 <br />700 <br />600 <br />500 <br />400 <br />300 <br />200 <br />100 <br />1900 1910 1920 1930 1940 1950 1960 <br />WATER YEAR <br />Figure 6. Annual transbasin exports from Upper Colorado River Basin, 1905 to 1983. <br />517,000 acre-ft/yr (U.S. Bureau of Reclamation, 1982; <br />J. Osterberg, U.S. Bureau of Reclamation, oral commun., <br />1986), which was about 3 percent of the virgin streamflow <br />of the Colorado River at Lees Ferry, Ariz., for the same <br />period. <br />The major effects of reservoirs on dissolved solids are <br />associated with evaporation, mixing and stratification in the <br />water body, and chemical processes in the water body. Net <br />evaporation from a water body removes water but leaves the <br />dissolved solids behind. This increases the concentration of <br />dissolved solids in the reservoir and ultimately in the water <br />released. The mixing of low- and high-concentration inflow <br />in a reservoir decreases seasonal variability in the outflow <br />concentration and also decreases the year-to-year variabil- <br />ity. However, stratification and density currents can limit <br />the mixing that occurs. <br />The effects of chemical reactions in a large reservoir <br />are difficult to quantify because of the problems of sampling <br />a large, dynamic volume of water at various depths, the lack <br />1970 1980 <br />of sufficient inflow data for accurate mass balance, and the <br />uncertainties associated with the modeling of geochemical <br />reactions in a deep, heterogeneous water mass. Buchak and <br />Edinger (1982) reported that the simple mixing of two waters, <br />such as lake water and stream inflow, could cause calcite <br />to precipitate in order to maintain chemical equilibrium. <br />However, they concluded that the potential decrease in <br />dissolved-solids load due to calcite precipitation in Lake <br />Powell would be negligible in comparison to the total an- <br />nual load. Several studies have indicated that precipitation <br />of calcite and silica does occur in large reservoirs along the <br />Colorado River (Smith and others, 1960, p. 185; Bolke and <br />Waddell, 1975, p. 11). <br />The effects of leaching from the banks of a reservoir <br />also are difficult to quantify. When a reservoir is drawn <br />down, water from bank storage may reenter the reservoir. <br />This water may have a high concentration of dissolved solids <br />if it has been in contact with soluble minerals in the bank <br />material. <br />10 Characteristics and Trends of Streamflow and Dissolved Solids in the Colorado River Basin