Sediment data of some form have been collected by the U.S. Geological Survey at 118 stream-gaging sites throughout Wisconsin, beginning in 1935. The average concentration of suspended sediment for Wisconsin streams is low in comparison with that of many streams in the United States—110 milligrams per litre for Wisconsin, as compared with 600 milligrams per litre for 50 percent of the United States (Rainwater, 1962). Enough data have been collected at 84 of the 118 sites to calculate an average annual suspended-sediment yield at those sites.
Measured average annual yields range from about 680 tons per square mile (238 tonnes per square kilometre) in the "Driftless Area" to 3.1 tons per square mile (1.1 tonnes per square kilometre) in the Northern Highland province. The average suspended-sediment yield for Wisconsin is about 80 tons per square mile per year (28 tonnes per square kilometre per year). Only three areas in the State may be considered to have a sediment problem. They are sections of the "Driftless Area" of southwestern Wisconsin, the Lake Superior red-clay area, and areas of expanding urbanization in southeastern Wisconsin.
Sediment-yield prediction equations have been developed for the Northern Highland province, the Central Plain province, the Eastern Ridges and Lowlands province, and the "Driftless Area" (Martin, 1932 and Thwaites, 1956). These four equations make it possible to predict average annual suspendedsediment yields at any point on about 95 percent of the streams in the State. The prediction technique involves regression equations that relate average annual suspended-sediment yields to factors, such as topography, soils, land use and cover, stream hydraulics, and climatic conditions. The standard error of estimate for these equations, which only represents the accuracy of an estimated sediment yield at an ungaged site, ranges from 28 to 38 percent.
