LEGACY SEDIMENT IN LOW-ORDER HIGHLAND STREAMS

This presentation describes some of the results of recent research that included the investigation of the impact of nineteenth century forest clearance and agricultural activity on rates of soil erosion, sediment source, and rates of evacuation of nineteenth century mill pond legacy sediment in low-order Southern Appalachian drainage basins. Both past and current research initiatives conducted in the adjacent Piedmont have demonstrated the scope of human impact on both water and sediment pathways in fluvial systems at the landscape scale. The occurrence, potential for remobilization, and ecological and economic impact of legacy sediment has been well documented for the region. However, very little comparative investigations have occurred in Southern Appalachia, a region presently experiencing increasing urban and suburban development.

Five mill ponds were constructed in the 25.5 km² Bent Creek Experimental Research Forest during the nineteenth century. Two of the five mill pond sediment deposits were extensively investigated. Magnetic susceptibility was used to identify variations in sediment source. Field observations, laboratory analyses, and computer modeling were used to determine a late nineteenth century sediment yield and sediment delivery ratio, and twentieth century rate of mill pond evacuation.

The results indicate a progression of soil degradation that appears to be linked with major precipitation events. Secondly, that the remobilization of a portion of mill pond sediment had a significant impact in terms of delaying the sedimentological recovery of Bent Creek during the twentieth century, the basin exporting substantial amounts of sediment at a time of aggressive hillslope stabilization efforts. However, at least one third of mill pond sediment remains in storage in the basin, and if we assume that the nineteenth century occupation and land use of Bent Creek was not atypical of Southern Appalachia then by extrapolation we may predict similar sediment storage in like basins. Of interest to future investigation of sediment pathways is how might potential changes in regional stream discharge, the possible result of continued urbanization, impact rates of mill pond sediment evacuation in a region that has until now not been viewed as a significant source of legacy sediment?