The 2760-hectare Big Eau Pleine Reservoir in Marathon County, Wisconsin has experienced frequent winter fish kills and summer algae blooms since its construction in 1937. From 1974 to 1979 a study of the reservoir and its 945-km2 watershed was conducted in an attempt to identify and quantify the sources of water quality problems and recommend management practices to reduce these problems. Land use and nutrient loading studies in the watershed identified agricultural runoff, especially animal waste, as the major source of nutrient loading. Total phosphorus loss from the watershed averaged 0.59 kg/ha/yr for the 4-year period with approximately 60 percent occurring during the spring snowmelt and runoff season. Hydrologic and soil erosion modeling indicated that the greatest runoff and soil erosion occurred during spring snowmelt and that much of the erosion and runoff originates on the lower slopes and alluvial soils. Reservoir studies identified Aphanizomenon flos-aquae as the major bloom producing alga. Chlorophyll a values for the four summers averaged 105 microgram/l, ranging from 65 in 1978 to 120 in 1976. Yearly variations in chlorophyll a did not correlate with differences in yearly or seasonal total phosphorus loading from external sources. Internal phosphorus loading appeared to be more important in determining summer algae blooms; much of the internal loading is believed to be from drawdown related resuspension of sediments. Total phosphorus levels in the reservoir begin to increase at about the same time as summer drawdown begins. Winter oxygen problems in the reservoir were related closely to reservoir drawdown. Sediment oxygen demand, long-term BOD studies, and reservoir monitoring showed that while considerable oxygen was lost over winter due to biological reactions, the reservoir would not go anaerobic. Winter drawdown was found to result in scouring of sediments high in BOD as the reservoir was gradually drawn down to the old river channel. This scouring resulted in rapid loss of the remaining oxygen as drawdown moved progressively down the reservoir. Recommendations include controlling animal-waste spreading during winter and increased use of conservation practices, especially on lower slope portions of the watershed, including fencing streams at least 30 feet from the stream channel. Recommended reservoir management changes include delaying summer drawdown to minimize internal phosphorus loading, delaying winter drawdown to at least mid-January, and increasing minimum pool volume by 25 percent.