Intensive row-crop agriculture dominates land use in the northwestern Ohio River basins draining into Lake Erie. Detailed, long-term studies of water quality in these rivers reveal high unit-area P loads entering Lake Erie, even though gross erosion rates are relatively low. Concentrations of nitrate-N and currently used herbicides are also high from May through July, both in surface waters and in public water supplies derived therefrom. Conservation tillage could increase nitrate and herbicide contamination of area waters. The region's detailed baseline nutrient and herbicide data provide an opportunity to evaluate the effects of conservation tillage on a variety of water quality parameters. To separate weather-related from management-related effects, an ecosystem approach is advocated. (Copyright (c) Soil Conservation Society of America 1985.)