Steiner Branch basin in southwestern Wisconsin has rugged mature topography. Corn is planted in 30 percent of the basin on slopes ranging from 0 to 20 percent. Although contour stripcropping is a recommended practice for these easily eroded soil slopes, few conservation practices are followed to reduce soil losses. Because the stream drains into a manmade lake used for recreation, its water quality is of major concern. The purpose of this report is to assess the magnitude and types of nonpoint discharges that affect the water quality of Steiner Branch.

Total stream discharge for the 1978 and 1979 water years was 1,500 cubic feet per second per day and 1,800 cubic feet per second per day, respectively. The 1978 water year discharge was about 90 percent of the average and the 1979 discharge was 120 percent of average. During the 1978 water year, base flow was about 60 percent of the stream discharge, and in 1979 it was about 78 percent. Streamflow during the 2-year study period ranged from 1.5 cubic feet per second, which is approximately the low flow that occurs on the average of once every 2 years, to 392 cubic feet per second, a discharge of about a 5-year flood-recurrence interval.

Suspended-sediment yields were 369 tons per square mile in the 1978 water year and 84.6 tons per square mile in 1979. These yields were 1.66 times higher than those monitored in an adjoining basin where more typical conservation practices were employed. However, suspended-sediment yield per unit of stream discharge was only 1.30 times higher in the Steiner Branch basin than in the adjoining basin. The estimated long-term annual suspended-sediment yield for the Steiner Branch basin is 444 tons per square mile. Sediment concentrations in Steiner Branch ranged from 3 to 6,430 milligrams per liter.

Most of the nutrient load of the stream was transported during runoff: total organic nitrogen, 80 percent; ammonia nitrogen, 80 percent; total phosphorus, 84 percent; and total orthophosphorus, 77 percent. Transport of nitrite plus nitrate nitrogen and total nitrogen occurred primarily during baseflow conditions, with 75 and 56 percent, respectively, of the total load for the study period being transported during these conditions. The time distribution of total phosphorus, total orthophosphorus, ammonia nitrogen, and total organic nitrogen transport was very similar to suspended-sediment transport in Steiner Branch.