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Four reservoirs in Phillips and Valley Counties were studied from May to August 1981 to describe: (1) the variation in water quality that occurs from late spring (May) to late summer (August) and during 24 hours in late summer (August), and (2) possible causes for water-quality variation during these periods. The reservoirs represent considerable ranges of water quality and reservoir age that exist for many similar reservoirs in eastern Montana. Although there are similarities in water quality of reservoirs from the same regional mineralogy, differences in water quality caused by local mineralogy and land use mask differences caused by reservoir age. All the reservoirs had distinct thermal gradients but lacked true stratification as a result of circulation induced by the wind. Such mixing helps prevent oxygen depletion in bottom waters during the summer. Dissolved-oxygen gradients existed in all the reservoirs, with the smallest concentrations occurring in the near-bottom water of the reservoirs and during the night. Nighttime dissolved-oxygen concentrations in reservoirs 19 and 24 were less than in reservoirs 1 and 9. Similar trends occured with pH. The fluctuations in dissolved-oxygen concentration and pH most likely result from variation in phytoplankton and zooplankton concentrations detected in conjunction with decomposition of organic matter in the bottom sediments. Most chemical constituents in the water generally became more concentrated from May to August because of cumulative water losses through evapotranspiration. However, this condition was most evident with the major dissolved constituents. Reservoir 19 had sodium bicarbonate sulfate water, reservoirs 1 and 24 had sodium bicarbonate water, and reservoir 9 had sodium sulfate water. Major dissolved constituents and nutrients had little change in concentration during diel sampling. Results of biological analyses were variable. All four reservoirs contained similar types of planktonic organisms but in different proportions. A common pattern for phytoplankton concentrations was a slight increase between the June and July sampling and a decrease from the early to late August sampling. The total number of benthic invertebrates were fewer in reservoir 9 than in reservoirs 1, 19, and 24. However, reservoir 9 had a more diverse benthic community than the other reservoirs. There were no consistent trends in bacterial or zooplankton concentrations among the study reservoirs. Major water-quality changes that occurred in the reservoirs could affect proposed uses of the study reservoirs. August concentrations of lead and nighttime concentrations of dissolved oxygen in reservoirs 19 and 24 exceeded the criteria protective of fish. Late afternoon pH in all reservoirs approached or exceeded the maximum limits protective of fish propagation, waterfowl habitat, livestock watering, and recreational swimming. INTRODUCTION Twenty-four reservoirs (fig. 1) in Phillips and Valley Counties were sampled in 1978 and 1979 to characterize their water quality and to evaluate their suitability for fish propagation, waterfowl habitat, livestock watering, and recreational use. Most of the reservoirs are formed by earthen dams constructed by the U.S. Bureau of Land Management. Reservoir surface areas ranged from 0.2 to 146 ha and maximum reservoir depths ranged from 0.05 to 6.5 m. To evaluate each of the 24 reservoirs, water-quality data were collected once during each limnologically extreme period: late winter, late spring, late summer, and early autumn. The data then were compared to a set of criteria protective of each proposed use (Ferreira, 1983). Several of the reservoirs were suitable for all the proposed uses; a few of the reservoirs had water quality that indicated unsuitability for all uses. All the reservoirs generally were enriched with nutrients (eutrophic) and, therefore, had the potential for massive algal blooms and suppressed dissolved-oxygen concentrations (Ferreira, 1980; 1983). The resultant small dissolved-oxygen concentrations could be stressful to fish. Compared to other times of the year, early spring and late summer commonly are periods of greatest phytoplankton production in lakes and reservoirs, mostly because of increased nutrient availability after spring snowmelt and subsequent warm-water temperatures and long days in late summer (Reid and Wood, 1976). During the summer, thermal stratification and succession of plant and animal populations can augment the variability in chemical water quality. In enriched lakes, water quality can vary considerably during 24 hours as a result of changes produced by photosynthesis, respiration, and decomposition. To gain a better understanding of the variation, the U.S. Bureau of Land Management entered into a cooperative program with the U.S. Geological Survey in 1981 to investigate May through August and diel (24-hour) changes in water quality in selected reservoirs that had been sampled in 1978 and 1979. Data from this study supplement the quarterly information obtained in 1978 and 1979. Purpose and scope The purpose of this report is to describe for four selected reservoirs: (1) the variation in physical, chemical, and biological quality of water occurring approximately monthly from late spring (May) to late summer (August) and during 24 hours in late summer (August) of 1981, and (2) possible causes for water-quality variation during these periods. Although only a subset of the reservoirs sampled in 1978 and 1979 was sampled for this study, there is enough difference within the subset that observed changes in water quality would be representative of similar changes in the remaining reservoirs. The four study reservoirs were chosen pri-

Water-quality variability in four reservoirs in Phillips and Valley Counties, Montana, May through August 1981