The hydrology and water quality of Swatara Creek were studied by the U.S. Geological Survey in cooperation with the Pennsylvania Department of Environmental Resources, Bureau of State Parks, from July 1981 through September 1984. The purpose of the study was to determine the effects of anthracite-coal mining and other point and nonpoint sources on the water quality of a planned 10,500 acre-foot reservoir. The Swatara State Park Reservoir is planned to be used for recreation and drinking-water supply for the city of Lebanon and surrounding communities. Annual precipitation during 1982, 1983, and 1984 was about 8 percent below, near normal, and 29 percent above the long-term average, respectively. The average annual precipitation during a year with near-normal precipitation, the 1983 water year, was 47 inches at Pine Grove. Mean streamflows during 1982, 1983, and 1984 were about 15 percent below, 4 percent above, and 50 percent above the long-term average, respectively. The average streamflow to the planned reservoir area during the 1983 water year was about 220 cubic feet per second. Inflows to, and downstream discharge from, the planned reservoir wer poorly buffered. Median alkalinity ranged from 4 to 7 mg/L (milligrams per liter) and median acidity ranged from 2 to 5 mg/L at the three sampling locations. Maximum total-recoverable iron, aluminum, and manganese concentrations were 100,000, 66,000, and 2,300 micrograms per liter, respectively. During 1983 the annual discharges of total-recoverable iron, aluminum, and manganese to the planned reservoir area were estimated to be 692, 300, and 95 tons, respectively. About 87 percent of the total-recoverable iron and 91 percent of total-recoverable sluminum measured was in the suspended phase. The data indicated that mine drainage affects the quality of Swatara Creek and will affect the quality of the planned reservoir. In addition to mine drainage, point-source nutrient and metal discharges will probably affect the planned reservoir. For example, in September 1983, Swatara Creek was sampled downstream from a point source. A dissolved- phosphorus concentration of 14 mg/L and total ammonia plus organic nitrogen concentration of 8.2 mg/L were measured. At the same location, concentrations of total-recoverable aluminum, chromium, copper, iron, and lead were 35, 300, 110, 1,300, and 32 micrograms per liter, respectively. Inflows to the planned Swatara State Park Reservoir are estimated to be acidic and rich in nutrients and select metals. Unless an effort is made to improve the quality of water from point and nonpoint sources, these conditions may impair the planned uses for the reservoir. Conservation releases from the reservoir need to be carefully controlled or these conditions also may degrade the water quality downstream.