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" ..., ,.-{ \ !~ 0 OJ 1. Thermal Stratification and Mixing Patterns Thermal stratification is one of the most impor- tant physical processes in the annual cycle of a lake or reservoir. Thermal stratification is a direct result of heating by the sun and the variability of water density with temperature. Typically, reservoirs such as Pueblo Reservoir that are located in the temperate zone undergo an annual cycle of warming and cooling that affect the timing and extent of stratification and mixing. In Pueblo Reservoir, thermal stratification results in a warm, less dense water layer near the reservoir sur- face-the epilimnion; a cooler, more dense deep layer-the hypolimnion; and a transitional zone between the two layers-the thermocline or metalim- nion, The density gradient that results from thermal stratification suppresses the vertical movement of water particles while allowing their horizontal move- mentto become more pronounced and persistent (Wunderlich, 1971). Stratification and mixing patterns in Pueblo Res- ervoir were evaluated with temperature-profile mea- surements that generally were made at 3-ft-depth increments from the water surface to the reservoir bot- tom at several established transects located between the reservoir inflow and the dam (fig. ]) from 1985 through ]989 (Ugland and others, ]988, 1990; Edelmann and others, 199]), The temperature-profile data indicate that there is little lateral variation in temperatures within a transect; therefore, the thermal profile of the reservoir may be adequately defined with measure- ments made at a central location within each transect. Additionally, diel variations in the water-temperature profile were evaluated with temperature data collected during separate 24-hour periods in July 1986, May 1987, and July ]987 (Edelmann and others, ]991), During these periods, reservoir temperature profiles did not vary enough to result in diel variations in reservoir stratification and mixing patterns; therefore, a single temperature profile made during the day generally can be expected to define the daily mixing and stratification patterns. A comparison of temperature-profile data col- lected from ] 985 through 1989 (U gland and others, 1988, 1990; Edelmann and others, 1991) indicates there is little year-to-year variation in the spatial and temporal temperature patterns in Pueblo Reservoir. Temperature-profile data were collected for a 5-year period that included a wide range of hydrologic condi- tions (figs. 2 and 3); therefore, unless Arkansas River or Pueblo Reservoir water-operations practices change substantially, the annual stratification and mixing pat- terns observed in ] 985 through 1989 can be expected to continue in the future if water levels, inflows, and out- flows remain within the range observed during 1985 through ] 989. The temperature profiles (fig, 4) also were used to evaluate the initial routing of the Arkansas River within the reservoir-the inflowing water enters the reservoir at a depth of equal density, which for Pueblo Reservoir generally is determined by water tempera- ture. Inflow that is wanner than the reservoir will enter as overflow at the reservoir surface because the warm inflow is less dense than the colder reservoir water. ]nterflow, which is the routing of water into the middle of the water column, results when the inflow is cooler and more dense than the surface water and warmer and less dense than the bottom water. Underflow results when the inflow is colder and more dense than the res- ervoir water and results in initial routing of the inflow along the reservoir bottom. The point in a reservoir where inflow plunges or flows beneath the reservoir surface is referred to as the plunge point. Underflow and interflow seem to be the more dominant flow patterns in Pueblo Reservoir. The plunge point in the reservoir typically is located in the upstream end of the reservoir between transects I and 3. Routing of inflow through the entire reservoir can- not be determined by the temperature of the inflow and upstream end of the reservoir because of some degree of mixing in the downstream end of the reservoir. The outlet type and operation of a reservoir also affect the routing of horizontal density currents, As previously discussed in the "]ntroduction" section, Pueblo Reser- voir has a mu]tilevel outlet structure that is capable of releasing water from several depths. However, the majority of flow is released from the river outlet located about 41 ft from the reservoir bottom. During periods that a reservoir is stratified, Ford (1990) indi- cated that vertical flow can be inhibited and the outflow zone can be restricted to a horizontal layer that can extend the full length of the reservoir. In Pueblo Res- ervoir, the horizontal layer encompasses a large portion of the reservoir and, during peak outflow, extends most of the length of the reservoir. Diel variations in the temperature of the Arkan- sas River result in corresponding diel variations in the initial routing of inflow into the reservoir. The mean diel variation in the temperature of the Arkansas River inflow is about4.50C, and the ranges of water temper- ature in the river often overlap the water temperature in the reservoir. Therefore, throughout much of the year, it is common for the initial routing of inflow to include overflow, interflow, and underflow during a 24-hour period, During these 24-hour periods, there still can be a dominant direction of inflow routing if the relative density difference between the river and reservoir 12 Physical, Chemical, and Biological Characteristics of Pueblo Reservoir, Colorado, 1985-89