The Onion River in eastern Wisconsin was studied during the 1979 and 1980 water years to define the relationship between water quality and streamflow. Agricultural nonpoint-source discharges in the lower part of the Onion River are suspected of contributing significantly to degradation of water quality.

Two streamflow water-quality monitoring stations were established on the Onion River, one at Hingham upstream of the affected area, and one near Sheboygan Falls downstream of the affected 272 pounds per square mile upstream of Hingham in the 1979 water year. Part of the high nutrient yields are due to point sources from Belgium Creek, which drains an area of 16.2 square miles. In the 1979 water year, point sources contributed an estimated 16,700 pounds of phosphorus or 45 percent of the total annual load downstream of Hingham, whereas, upstream of Hingham point sources contributed an estimated 2,200 pounds of phosphorus, or 18 percent of the total annual load. At Hingham during base flow, four of five phosphorus concentrations exceeded levels recommended by the U.S. Environmental Protection Agency; most concentrations during storm runoff exceeded these levels. At Sheboygan Falls, all phosphorus concentrations at base flow and most storm runoff concentrations also exceeded the Environmental Protection Agency criteria. All samples at both Hingham and Sheboygan Falls contained concentrations of un-ionized ammonia that were less than the U.S. Environmental Protection Agency's criteria (0.02 milligrams per liter), except the March 1980 samples. No samples exceeded the Wisconsin Department of Natural Resources criteria for un-ionized ammonia (0.04 milligrams per liter). area. Streamflow at Onion River at Hingham ranged from a minimum 7-day mean low flow (Q?) of 8.7 to a maximum discharge of 600 cubic feet per second; at Onion River near Sheboygan Falls streamflow ranged from a Q7 of 13 to a maximum discharge of 2,350 cubic feet per second. Based on discharges at the Sheboygan River at Sheboygan, these discharges ranged from about double the low flow that occurs on the average of once every 2 years, to discharges of between 5-and 10-year recurrence intervals. The average discharges at Hingham in the 1979 and 1980 water were 32.2 and 27.6 cubic feet per second, respectively. At Sheboygan Falls, the average discharges in the 1979 and 1980 water years were 94.2 and 55.2 cubic feet per second, respectively. Based on the average discharge at the Sheboygan River at Sheboygan, the 1979 discharge was 60 percent greater than average, and the 1980 discharge was about 5 percent less than average.

Precipitation in the study area for the 1979 water year was 33.1 inches and for the 1980 water year it was 36.8 inches; these amounts are 3.3 inches and 7.0 inches, respectively, more than average.

Suspended-sediment yields and, probably, phosphorus yields were slightly above average for both years because of greater than normal precipitation, and stream discharges that were greater than normal in 1979 and near normal in 1980. Suspended-sediment yields were 79.1 tons per square mile for the 1979 water year and 63.9 tons per square mile for the 1980 water year at Hingham, while downstream of Hingham the yields were 93.5 tons per square mile for the 1979 water year and 84.2 tons per square mile for the 1980 water year. Phosphorus yields were 331 pounds per square mile for the 1979 water year and 317 pounds per square mile for the 1980 water year at Hingham. Downstream of Hingham, the phosphorus yields were 656 pounds per square mile for the 1979 water year and 647 pounds per square mile for the 198Q water year.

A population of bottom-dwelling carp resuspends the bottom sediments during its late spring and early summer active period, possibly causing high concentrations of suspended sediment and phosphorus.

Nutrient yields and loading rates were highest downstream of Hingham. Nonpoint-source contribution of phosphorus amounted to 362 pounds per square mile downstream of Hingham compared to 272 pounds per square mile upstream of Hingham in the 1979 water year.

Part of the high nutrient yields are due to point sources from Belgium Creek, which drains an area of 16.2 square miles. In the 1979 water year, point sources contributed an estimated 16,700 pounds of phosphorus or 45 percent of the total annual load downstream of Hingham, whereas, upstream of Hingham point sources contributed an estimated 2,200 pounds of phosphorus, or 18 percent of the total annual load.

At Hingham during base flow, four of five phosphorus concentrations exceeded levels recommended by the U.S. Environmental Protection Agency; most concentrations during storm runoff exceeded these levels. At Sheboygan Falls, all phosphorus concentrations at base flow and most storm runoff concentrations also exceeded the Environmental Protection Agency criteria.

All samples at both Hingham and Sheboygan Falls contained concentrations of un-ionized ammonia that were less than the U.S. Environmental Protection Agency's criteria (0.02 milligrams per liter), except the March 1980 samples. No samples exceeded the Wisconsin Department of Natural Resources criteria for un-ionized ammonia (0.04 milligrams per liter).