Eau Galle Reservoir, located in west central Wisconsin, is a small eutrophic flood control impoundment. The reservoir sustains high sediment and associated nutrient loadings from its agricultural watershed. Despite this system's riverine nature, much of the phosphorus accumulated in the water column during the summer derives typically from internal loading. Entrainment of phosphorus in the epilimnion from hypolimnetic sources during weather-induced partial mixing events has encouraged the development of large phytoplankton blooms. In 1986 internal phosphorus loading was substantially reduced by applying alum to profundal sediments. However, this effect was short-lived (< 1 year) because of high rates of sedimentation with probable burial of alum. Phytoplankton production was not diminished by alum treatment, partly because of high rates of external phosphorus loading during the unusually wet summer of 1986. In addition to inputs from the anaerobic hypolimnion, phosphorus is also transported to surface waters from the littoral zone by convective exchange flows driven by diel changes in water temperature. Phosphorus release from littoral sediments is enhanced at elevated values of pH, induced by macrophyte and phytoplankton photosynthesis. Groundwater discharge may provide another source of phosphorus to the epilimnion of Eau Galle Reservoir.Ceratium hirundinellaand other dominant phytoplankton species (blue-green algae) possess significant vertical migrational capabilities, and are thus able to maximize phosphorus uptake whether from the hypolimnion, the littoral zone, or directly from riverine inputs. As with many other eutrophic reservoirs, it is difficult to control nutrient inputs from external sources in Eau Galle Reservoir. Control of internal sources of phosphorus loading to this reservoir appears to be equally difficult.