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2343 August. The August 1987 dissolved-oxygen profile indicates concentrations also were nearly depleted in the downstream part of the reservoir near the ther- mocline. The settling rate of organic matter, phy- toplankton, and zooplankton in reservoirs is slowed when it encounters the more dense metalimnetic water, thus, potentially allowing more time for oxygen- consuming decomposition and respiration (Gordon and Skelton, 1977). By October, the reservoir undergoes fall turnover, thus eliminating strong thennal stratifica- tion and redistributing dissolved oxygen throughout the water column. A decrease in photosynthesis rates in the colder months results in dissolved-oxygen con- centrations less than saturation and pH values that vary little with depth. Relatively small diel variations in dissolved- oxygen concentrations and pH occur in Pueblo Reser- voir during the summer (Edelmann and others, 199 I). Diel variations that do occur in reservoirs have been attributed to dief changes in rates of photosynthesis and respiration within the water column (Goldman and Horne, 1983). During daylight hours, photosynthesis typically results in increases in dissolved-oxygen con- centrations and pH within the euphotic zone. During nighttime hours, photosynthesis ceases and respiration results in decreases in dissolved oxygen and pH in the euphotic zone, Dissolved Solids Dissolved solids is a measure of the quantity of dissolved minerals in water and is used as a measure of inorganic water quality. Large concentrations of dissolved solids can adversely affect water quality for irrigation and municipal use. The major dissolved sol- ids in Pueblo Reservoir include the major ions: cal- cium, magnesium, sodium, potassium, bicarbonate, sulfate, and chloride. A statistical summary of dissolved solids and major-ion concentrations in the upstream (site 3B), middle (site 5C), and downstream (7B) parts of Pueblo Reservoir is provided in table 2. The concentrations of dissolved solids in Pueblo Res- ervoir can be affected by the concentration of dissolved solids entering the reservoir from the Arkansas River, reservoir evaporation, diffusion of bottom-sediment pore water, and geochemical controls on mineral solu- bilities. Concentrations of most water-quality constitu- ents flowing into Pueblo Reservoir can be estimated with data collected from the Arkansas River about 10 mi upstream from the reservoir (station 07097000, Arkansas River at Portland); this is not the case for dis- solved solids because dissolved-solids concentrations increase by about I 0 percent between station 07097000 and the reservoir. Dissolved-solids samples collected at sites I Band 7B in 1986 and 1987 were used to esti- mate concentrations of dissolved solids in the reservoir inflow and outflow, respectively, The median dissolved-solids concentrations for sites] Band 7B during 1986 through 1987 were 224 mgIL and 262 mgIL, respectively, A statistical analysis of the dissolved-solids concentrations using the Mann- Whitney test indicates the increase in dissolved-solids concentrations is not significant (p-value = 0.28), The major-ion composition of water samples from the upstream, middle, and downstream parts of Pueblo Reservoir (sites 3B, 5C, and 7B) are very simi- lar to one another (table 2). Calcium is the dominant cation, and bicarbonate and sulfate are the codominant anions (based on milliequivalents per liter) throughout the reservoir. Relations of dissolved-solids and major- ion concentrations to specific conductance were devel- oped for Pueblo Reservoir (table 3). The relations were developed using least-squares regression with a com- posite of all data collected at sites 3B, 5C, and 7B dur- ing ] 986 through 1989. All of the equations are significant at the 99-percent confidence level and have coefficients of determination (,2) greater than 0.65, which indicates that dissolved-solids concentrations and individual major-ion concentrations may be accu- rately estimated with specific-conductance measure- ments. A comparison of major-ion concentrations for water samples collected near the reservoir surface and near the reservoir bottom indicates that concentrations of samples collected near the reservoir bottom gener- ally are larger than those collected near the surface. However, the vertical variability in most major-ion concentrations generally is not statistically significant when measured with the Mann-Whitney statistical test (95-percent confidence). Differences between calcium concentrations at the surface and bottom at site 7B and bicarbonate concentrations at the surface and bollom at site 3B were significantly different. Geochemical con- trols might affect the concentrations of calcium within the water column. Modeling of water analyses with the geochemical model WATEQ4F (Ball and Nordstrom, 1991) indicates much of the reservoir typically is supersaturated with calcite, the major source of cal- cium, throughout the year. Quite often during the sum- mer when pH values near the reservoir bottom decrease to less than about 7,8, calcite is undersaturated and may result in dissolution of calcite; therefore, calcium con- centrations near the reservoir bottom often are signifi- cantly larger than near the surface. ]n other reservoirs, larger bicarbonate concentrations near the reservoir bottom than near the surface have been indicated to be CHEMICAL CHARACTERISTICS 27