Human Impacts on Wisconsin Stream Channels∗

The conversion of natural land cover to agricultural land uses in the Platte watershed of southwestern Wisconsin caused a three to fivefold increase in the magnitudes of floods that recur more frequently than once in five years in the small headwater and tributary watersheds. The impact of settlement on floods is less for larger watersheds, but the general impact of settlement has caused a major channel metamorphosis throughout the Platte River channel system. Most headwater and tributary channels have modern channel cross sections that are relatively wide and shallow when compared to their presettlement counterparts. Most channel cross sections along the downstream main channel, beyond the channel reaches where bedload sediments are a dominant factor in determining channel shape, are narrow and deep relative to their presettlement counterparts. Much of the sediment that is related to man-induced accelerated surface runoff has been transported only short distances and is presently deposited on alluvial fans and floodplain surfaces. Historical floodplain alluviation varies in thickness from about one and one-half feet (0.5 m) in the headwaters to about twelve feet (4 m) near the mouth of the river. The morphology of channels and the mobility and storage of sediments in the Platte watershed responded quickly to environmental changes in the watershed. Notes ∗I am grateful to Mr. Walter E. Scott of Madison, Wisconsin, for providing many historical documents that would have otherwise been unavailable and for suggesting many useful sources of historical data. I am also indebted to William Corcoran of Michigan State University and Katherine Hirschboeck of the University of Arizona who provided field assistance during their tenure as undergraduates at the University of Wisconsin. I am especially indebted to William Johnson of the University of Oklahoma and to Patrick Bartlein of the University of Wisconsin who helped with data collection during the latter stages of the study and who provided many fruitful discussions during the development of this manuscript. The study was partly supported by the National Science Foundation, Grant GI-29731.