CHANNEL SOURCES AND STORAGE OF LEGACY SEDIMENT IN AGRICULTURAL STREAMS AND IMPLICATIONS FOR SEDIMENT AND PHOSPHORUS LOADINGS AND BIOLOGICAL IMPAIRMENTS, DRIFTLESS AREA, WISCONSIN

A targeted phosphorus reduction project in the Pecatonica River headwaters in the southwest Wisconsin Driftless Area is examining the role of channel processes and legacy sediment in the delivery of phosphorus and sediment to the Upper Mississippi River. This research is part of a larger study by the University of Wisconsin-Madison, University of Wisconsin-Platteville, The Nature Conservancy, Dane and Lafayette Counties, Wisconsin Department of Natural Resources, and the U.S. Geological Survey designed to examine the spatial and temporal pathways of phosphorus and sediment from fields through channel networks and eventual export at a watershed outlet. In 2009-10, rapid channel assessments were completed at 75 sites across three small watersheds including quantitative measurements of bank erosion and fine-sediment deposition, with channel cross sections, pebble counts, and biological assessments done at a subset of sites. Sites were spatially distributed throughout the watershed from ephemeral headwater channels to perennial main stems to estimate watershed sediment budgets. Preliminary assessment results show that average fine-sediment deposition in 2^nd and 3^rd order channels can be over 30 cm thick where channel slopes are 0.3 percent or less. For the small study watersheds, the volume of fine sediment deposited and stored in channels is roughly equal to about 10 years worth of the estimated sediment losses from upland soil or stream-bank erosion. Since the 1950s soil erosion has been substantially reduced in the Driftless Area by widespread adoption of land conservation practices such as contour-strip cropping and conservation tillage, and increased forest cover and native grass cover on hill slopes. However, many miles of streams in the area are on the state’s impaired waters list due to sediment-related habitat degradation. Stored historical fine sediment in valley bottoms, banks, and on the streambed likely contribute a substantial portion of the suspended sediment and phosphorus loading to the upper Mississippi River watershed.