LAND USE IMPACTS ON FLUVIAL PROCESSES IN THE NEMADJI RIVER WATERSHED

The Nemadji River drains 1100 km2 of eastern Minnesota and northwestern Wisconsin. Channel incision and mass wasting are natural responses to glacial rebound in this area and account for more than 95 % of the annual sediment load. However, the clay and sand delivered by the Nemadji to Lake Superior has increased over the past 150 years. We researched land use history across the upper 520 km2 of the Nemadji River Watershed, surveyed channel characteristics, identified relic channels and employed dendrochronology to date floodplains and terraces. Results indicate that two episodes of channel incision began propagating through the Nemadji River and its tributaries. One associated with timber harvesting in the mid 1800's and another associated a large fire in 1894. Streams impacted by incision had increased slope, reduced sinuosity, increased entrenchment, and reduced width depth ratios. Subsequent erosion from steep banks, mass wasting, and upstream incision provides sediment and space needed for the reconstruction of stable channels. The new channels are similar to the relic channels and un-impacted streams. Historical increases in water yield, particularly bankfull discharge, initiated the channel incision. This occurred with the onset of basin scale land use conversion in the 1850's. Forest fires in 1894 and 1918 and agricultural land use conversion during the early 1900's initiated additional episodes of channel incision