Water samples were collected from 12 reservoirs in northeastern Montana during the late winter, spring, summer, and early fall of 1978. The resulting physical and chemical water-quality data will be useful in managing the reservoirs for use of stock watering, waterfowl propagation, fish production, and recreation. The surface areas of the reservoirs range for 0.4 to 28.1 hectares with water depths ranging from 0.25 to 6.9 meters. The chemical composition of water varied among reservoirs and with season. Specific conductance ranged from 446 to 4,020 micromhos per centimeter during the winter sampling period and from 62 to 999 micromhos per centimeter during the spring sampling period. Although a pH of 6.9 was measured at one reservoir during the winter, pH values generally ranged from 7.7 to 9.8. Under ice cover, which averaged 1.0 meter thick in late February, three reservoirs attained dissolved-oxygen concentrations above saturation. Each reservoir can be classified as having one of the following three water types: sodium bicarbonate, sodium sulfate, or calcium sodium bicarbonate.
