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2321 RECONNAISSANCE OF WATER QUALITY OF PUEBLO RESERVOIR, COLORADO--MAY THROUGH DECEMBER 1985 By Patrick Edelmann ABSTRACT Pueblo Reservoir is the farthest upstream, main-stem reservoir con- structed on the Arkansas River and is located in Pueblo County approximately 6 miles upstream from the city of Pueblo. This reconnaissance study provides a better understanding of the reservoir and the quality of water entering Pueblo Reservoir and makes a preliminary assessment of some of the water- quality characteristics of the reservoir. During the 1985 sampling period, the reservoir was stratified, and under- flow from the Arkansas River occurred that resulted in stratification of the water in the reservoir with respect to specific conductance. Concentrations of dissolved solids decreased markedly below the thermocline during June. Later in the summer, dissolved-solids concentrations increased substantially below the thermocline. The variations in specific conductance that occurred in the reservoir generally coincided with variations in specific conductance measured at the Arkansas River at Portland. Substantial depletion of dis- solved oxygen occurred near the bottom of the reservoir. The dissolved oxygen minimum of 0.1 milligram per liter occurred during August near the reservoir bottom at transect 7 (near the dam). At the reservoir surface, total- inorganic-nitrogen concentrations averaged about 0.2 milligram per liter and total-phosphorus concentrations ranged from less than 0.01 to 0.05 milligram per liter. At the reservoir bottom, total-inorganic-nitrogen concentration averaged 0.3 milligram per liter, and total-phosphorus concentrations ranged from less than 0.01 to 0.22 milligram per liter. Concentrations of most trace elements were small and were less than the established State water-quality standards for Pueblo Reservoir. However, concentrations of total iron occa- sionally exceeded the aquatic-life standard near the reservoir bottom at transect 2, and dissolved-manganese concentrations occasionally exceeded the standard for public water supply near the reservoir bottom at transect 2. Diatoms, green algae, blue-green algae, and cryptomonads comprised the majority of the phytoplankton in Pueblo Reservoir in 1985_ The maximum con- centration of phytoplankton, average of 41,000 cells per milliliter, occurred in July. Blue-green algae dominated from June to September; diatoms were the dominant group of algae in October. The average concentration of phytoplank- ton decreased from July to October. 1