The upper Mississippi River basin from Royalton to Hastings, Minnesota, includes seven subbasins in east-central Minnesota that cover an area of 8,500 square miles. Results of a study, using data from the Minnesota Pollution Control Agency, Metropolitan Waste Control Commission, and the U.S. Geological Survey, indicate that selected water-quality characteristics differ significantly among subbasins. Results of the study also indicate that the quality of water leaving the basin at Hastings is affected primarily by inflow from the Minnesota River and by effluent from the Metropolitan sewage-treatment plant.

Subbasins in the western part of the study area are underlain by prairie soils and cultivation of row crops is a common land use. Streams draining these subbasins have a median dissolved-solids concentration of 389 mg/L (milligrams per Liter) and a median concentration of nitrite plus nitrate nitrogen of 0.59 mg/L. Subbasins in the northern and eastern parts of the study area are underlain by more acidic podzol soils. Land use in these subbasins is less devoted to cultivated crops; forested areas, pastures, and wetlands are common. Streams draining these subbasins have a median dissolved- solids concentration of 184 mg/L and a median concentration of nitrite plus nitrate nitrogen of 0.17 mg/L.

The quality of water changes dramatically in the most downstream subbasin, which includes the Twin Cities Metropolitan Area. On the basis of hourly data from automatic monitors, specific conductance increases from 345 /μS/cm (microsiemens per centimeter) at 25° Celsius above the confluence with the Minnesota River to 467 /μS/cm below it. Specific conductance increases to a median of 513 /μS/cm where effluent from the Metropolitan sewage-treatment plant enters the Mississippi River.

Dissolved-oxygen concentrations in the Mississippi River begin to decline below the confluence with the Minnesota River. Concentrations of dissolved oxygen reach a minimum median value in summer of 6.3 mg/L at a point about 9 miles downstream from the Metropolitan sewage-treatment plant. In winter, minimum median concentration is downstream at Lock and Dam 2, which is approximately 20 miles below the plant.

Results of this study show that the quality of water in the Mississippi River as it leaves the accounting unit at Hastings is not representative of water quality in most of the accounting unit. Three water-quality regions have been identified, and sampling sites are needed in each region to assess the quality of streams throughout the study area adequately.