HISTORIC MILL DAMS AND GEOLOGIC SETTING INFLUENCE VERY SLOW RATES OF BANK EROSION ALONG THE BEDROCK-INFLUENCED SOUTH RIVER, VIRGINIA

Bank erosion has been implicated as an important ongoing source of mercury contamination to the South River, a gravel-bedded stream in the Shenandoah Valley of Virginia. Studies of aerial photographs from 1937, 1957, 1974, and 2005 in a GIS framework over a 37 km study reach indicate that the median rate of bank retreat from 1937-2005 is only 0.02 m/yr. The 95th percentile erosion rate is only 0.15 m/yr. 33% of the total volume of bank erosion is related to the development of islands, while 26% occurs at migrating bends. 15% of bank erosion occurs in straight reaches, 8% is directly associated with the failure of small mill dams, and 6% occurs near tributary confluences. Bank erosion rates increase with increasing bend curvature and decrease with increasing density of riparian vegetation, but geologic setting is the most important control: bank erosion rates are lowest where cutbanks are composed of alluvial terrace deposits or bedrock. Bank erosion rates also have increased with time. Increases in bank erosion rates from 1937 to 2005 cannot be explained by changes in storm discharges or riparian vegetation, but appear to be correlated with the loss of 13 mill dams along our 37 km study reach. High precision tripod mounted lidar surveys document patterns of bank erosion at smaller spatial and temporal scales than can be determined from aerial photographs.