Assessing cumulative effects in the Nemadji River Basin, Minnesota

The Nemadji River in northeastern Minnesota contributes an estimated 525,000 metric tons of sediment per year to Lake Superior. A portion of the Nemadji Basin is composed of highly erodible red clays that are susceptible to soil mass movement. The red clay area was studied in the late 1970s through a joint interagency project that concluded that natural levels of erosion-sedimentation in the red clay area have been intensified by human activity. The cumulative effects of logging, burning, clearing for agriculture and road construction are increased runoff, higher peak streamflows, rapid channel scouring, increased soil mass movement, and streambank undercutting. This study uses a Geographic Information System (GIS) to analyze watershed characteristics of subwatersheds within the Nemadji basin. Bivariate analyses indicated that frequency of slump occurrence was inversely related to total forested area. The results of this investigation indicate that the percent of nonforested area is related to a greater frequency of soil slumps, and that management should be aimed at increasing and maintaining forest cover in the watershed.