Investigation of Pre-Settlement Floodplain Conditions In Preparation for River Restoration In Southwest Wisconsin

The East Branch Pecatonica River in Southwestern Wisconsin is the location of an ongoing Wisconsin Department of Natural Resources and Nature Conservancy river restoration project aimed at reconnecting the river to its floodplain. Restoration efforts along a valley reach in 2006 involved excavation of overbank and channel margin fluvial sediment deposited in the mid-19th to early 20th centuries primarily due to poor soil conservation practices on highly erodable agricultural land. The removal of this sediment is intended to restore the morphology of the pre-agriculture floodplain and re-establish connectivity between the channel and its floodplain, thereby creating riparian habitat and facilitating improved nutrient and sediment storage.

Similar morphologic restorations are planned for an adjacent reach in 2008. Numerous sediment cores were extracted to map in detail the surface topography of the pre-agriculture valley floor and floodplain surface. This more accurate reconstruction of the pre-agriculture floodplain morphology will serve as a basis of comparison for evaluating how the level of accuracy in reconstructing pre-agriculture floodplain morphologies influences the success of meeting restoration objectives. Cesium-137 results suggest that deposition of fluvial sediments occurred throughout the 1950s and 60s on the site, and redeposition of sediment from the slopes occurred post-1963. One dimensional HEC-RAS modeling is used to evaluate the transport and storage efficiencies for the restored versus existing channel and floodplain morphologies. A rain storm exceeding the 100-year, 24 hour expected maximum magnitude occurred in August 2007. Runoff from this storm was mostly contained between the high banks of the historical meander belt in the unrestored valley reach. Consequently, flood runoff, sediment, and nutrients experienced reduced opportunity for storage in the unrestored reach and were in large part efficiently transported downstream.