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~4'" 1')'1 . ,,0 J than saturation throughout the reservoir, Dissolved- oxygen concentrations less than I mgIL commonly occur near the reservoir bottom downstream from about transect 5 to the dam in July through mid- September. The periods of low dissolved-oxygen con- centrations typically persist for less than 3 to 4 weeks. Completely anoxic conditions were observed in parts of Pueblo Reservoir on four occasions during 1985 through 1989. Anoxic conditions were measured at site 48 in August 1987, sites 5C and 78 in July 1988, sites 6C and 78 in August 1988, and at site 7B in Sep- tember 1989. Typically, the anoxic conditions were localized very near the reservoir bottom. The most extensive period of anoxia was during the summer of 1988. In August 1988, the bottom 12 ft of site 6C and the bottom 30 ft of site 78 were completely anoxic. An empirical evaluation of available outflow, storage, and dissolved-oxygen data indicates that the onset of depleted dissolved-oxygen concentrations in the hypolimnion is related to the amount of flow- through that occurs during the snowmelt period, which generally occurs during June, Cole and Hannan (1990) reported the following: (I) anoxia typically originates near the plunge point in the upstream end of many res- ervoirs and (2) anoxia occurs sooner during high flows because of more rapid displacement of cold hypolim- netic water with warmer water that has smaller dissolved-oxygen concentrations. Anoxia in Pueblo Reservoir does not originate in the upstream end near the plunge point because the inflow is cold and well oxygenated. Additionally, the inflow contains rela- tively little organic material and has a relatively small biochemical oxygen demand. During 1988, when hydraulic residence times were substantially longer than during 1985 through 1987 and 1989, low dissolved-oxygen concentrations occurred sooner, affected a larger part of the hypolimnion, and resulted in a larger anoxic zone. The onset of low dissolved- oxygen concentrations in Pueblo Reservoir typically occurs downstream from transect 5 and results from the impedance of mixing of deeper hypolimnetic water with the overlying oxygenated density currents of the inflow and outflow zones and the epilimnion. This mixing impedance limits reaeration of depleted hypolimnetic dissolved-oxygen concentrations that result from the decomposition of organic material in the water column and at the sediment-water interface of the reservoir bottom. The deeper hypolimnetic water can be more easily mixed or flushed as part of the out- flow by large inflows and outflows than by smaller flows. The pH of water in Pueblo Reservoir typically varies spatially and seasonally, and typically ranges from 7.5 to 9,0. Generally, pH values are largest near the reservoir surface and decrease with increasing depth. Variations in pH with depth decrease during well-mixed conditions and increase with the onset of strong thermal stratification. Dissolved-oxygen and pH-measurement profiles of Pueblo Reservoir for March, June, August, and October 1987 illustrate the typical spatial and temporal patterns of dissolved-oxygen concentrations and pH that occur annually (fig. 10). The March profile is rep- resentative of early spring conditions when the reser- voir is beginning to warm but is still generally well mixed, During this period, the primary effects on dis- solved oxygen and pH within the water column proba- bly are aeration of the surface water because of wind and wave action and oxygen consumption at the sediment-water interface because of decomposition of organic material and respiration. Some decrease in dissolved-oxygen concentrations occurs with depth apparently because of the limits of vertical mixing. The consumption of dissolved oxygen by the oxidation of organic material and the respiration of plants and animals occur at a faster rate than the oxygen can be supplied by vertical mixing or inti owing upstream water at depths below the euphotic zone (Wetzel, 1983). These oxidative process demands for dissolved oxygen are most intense at the sediment-water inter- face on the reservoir bottom, where settling organic material accumulates (Wetzel, 1983). The March pH profile indicates the water column was well mixed with respect to pH, and the only change in pH with depth occurred in the upstream part of the reservoir, The June profiles are representative of the early summer period when strong thermal stratification develops, and pri- mary productivity by phytoplankton is occurring at rel- atively high rates. Because of the strong thermal stratification, vertical mixing and redistribution of dis- solved oxygen within the water column are limited to the epilimnion, and dissolved-oxygen concentrations decrease substantially from the supersaturated cond,- tions at the reservoir surface to concentrations much less than saturation in the hypolimnion. During the summer, the pH of the water column decreases with depth. The consumption of carbon dioxide during pho- tosynthesis increases the pH in the epilimnion, whereas the release of carbon dioxide during respiration decreases pH in the metalimnion and hypolimnion (Wetzel, 1983), The August dissolved-oxygen profile indicates that supersaturated dissolved-oxygen concen- trations continue to occur in the epilimnion because of photosynthesis by phytoplankton. Dissolved-oxygen concentrations decrease with depth and are nearly depleted near the reservoir bottom. In 1985 through 1989, the dissolved-oxygen minimum generally occurred in the hypolimnion at transects 6 and 7 during 24 Phyalcal, Chemical, and Biological Characterlatics 01 Pueblo Reaervolr, Colorado, 1985-89