Four reservoirs in Phillips and Valley Counties were studied from May to August, 1981 (1) to describe the variation in water quality that occurs from late spring (May) to late summer (August) and during 24-hrs in late summer and (2) to describe possible causes for the variation. All the reservoirs had distinct thermal gradients but lacked true stratification as a result of circulation induced by wind. Such mixing helps prevent oxygen depletion in bottom waters during the summer. The smallest dissolved-oxygen concentrations occurred in the near-bottom water of the reservoirs and during the night. Nighttime dissolved-oxygen concentrations in reservoirs 19 and 24 were much less than in reservoirs 1 and 9. Similar trends occurred with pH. Most chemical constituents in the water generally became more concentrated from May to August because of continuous water losses from evapotranspiration. This was noted by increase in the concentration of major dissolved constituents and by increase in specific conductance. All the reservoirs contained similar types of planktonic organisms, but in different proportions. Most of the fecal bacteria existing in the reservoirs presumably originated from livestock and waterfowl waste. (USGS)
