Delavan Lake is a eutrophic, recreational lake in a densely populated area of southeastern Wisconsin. Sewage effluent and septic tank drainage were diverted out of the drainage basin of the lake to improve its water quality in 1981. The worst known blue-green algal bloom occurred in the lake in the summer of 1983. A comprehensive hydrologic and water-quality investigation was started in October 1983 to determine why the water quality in the lake apparently had not improved after cessation of influxes of waste waters. This report describes the hydrology and water quality of Delavan Lake and its drainage basin during the 1984 and 1985 water years.

All major external inputs and outputs of phosphorus and nitrogen to the lake were measured to assess the importance of each source. Internal loading of phosphorus was calculated as a residual of a mass-balance budget. The in-lake phosphorus mass during the 2-year study shows a gradual declining trend that suggests an improving lake condition but also could be due to a random phosphorus decline. Future monitoring will be needed to determine whether the decline in phosphorus has indeed occurred as a result of waste-water diversion.

Continuous streamflow and water-quality monitoring in the subbasins of the Delavan Lake watershed showed a wide range of annual phosphorus and nitrogen yields during the study. Jackson Creek tributary, which predominantly drains the city of Elkhorn, had the highest average yields of phosphorus (838 pounds per square mile) and Kjeldahl nitrogen (3,600 pounds per square mile). These yields were almost three times those found in the Jackson Creek basin, which has the largest livestock population. The other major tributary, Delavan Lake tributary 2, had much lower average annual yields of phosphorus (53.1 pounds per square mile) and Kjeldahl nitrogen (550 pounds per square mile). The lower yields in this tributary result from significantly reduced runoff caused by storage and evapotranspiration in a large pond that is surrounded by a wetland in the stream's basin.

External loading of phosphorus and nitrogen were sufficient to cause eutrophic conditions. Internal loading of phosphorus was more than two times the external phosphorus supply. Most of the internal loading occurred when the hypolimnion was anoxic during summer. Internal loading of phosphorus during the 1985 water year was significantly reduced from that of 1984 because of a shorter anoxic period.