THE SOUTH RIVER, VA: AN ALLUVIAL CHANNEL WITH HIGHLY STABLE RIVERBANKS

Long-term rates of bank erosion were determined by comparing shorelines mapped on orthorectified aerial photographs from 1937 – 2005m. The locations of presently eroding riverbanks were mapped in 2005 and 2006 by floating down the river in a canoe and noting their start and end points with a GPS. The riverbanks are usually stable: rates of lateral migration from 1937-2005 are barely resolvable on aerial photographs despite rectification errors that are typically less than 1m. Field surveys indicate that only 12.4% of the entire shoreline is currently actively eroding. Patterns of channel migration cannot be explained by existing models which rely on channel curvature as a fundamental control on channel migration: many very tight bends on South River have been essentially immobile from 1937-2005. Factors that contribute to the stable banks of South River include frequent bedrock exposures, abundant trees, and cohesive bank materials. When significant changes in bank positions occur, they seem to be related to several factors. Erosion mainly occurs where riverbanks coincide with the alluvial floodplain, rather than bordering bedrock outcrops. Some narrow secondary side channels around islands have widened and captured the main flow. In other cases, mid-channel bars are areas of significant bank erosion, as the bars deflect the main flow into the banks. Bank erosion also occurs in conjunction with tributary confluences, where deltas and confluence bars divert the main flow into the opposite bank, leading to significant retreat if banks are not protected by bedrock or riparian forests. Our observations indicate that new models of bank erosion and channel change are needed to guide river restoration and engineering projects along stable rivers such as the South River in our study area.