Increased Mid-20th Century River Bank Erosion Rates Related to the Demise of Mill Dams, South River, Virginia

A recent hypothesis suggests that fluvial processes in the eastern United States are driven by the demise of colonial mill dams, rather than reflecting a quasi-equilibrium adjustment of streams to current conditions. We evaluated the control of colonial mill dams on 20th and 21st century bank erosion rates through studies of historical aerial photographs, historical documents, hydrologic and climatic records, and hydraulic modeling. Historical sources and aerial photographs document 8 colonial mill dams along the study reach in the early 20th century; all but 1 of these dams disappeared in the 1950s, and the last was breached by 1976. From initially low values between 1937 and 1957, mean bank erosion rates increased by more than a factor of 2 after 1957, remaining high through 2005. Accelerated bank erosion rates cannot be explained by changes in storm intensity, the frequency of freezing-thaw cycles, or by changes in the density of riparian trees. Hydraulic modeling suggests that mill dams reduced velocities of the 5-year flood through approximately 80% of our study reach. By considering the timing of mill dam loss, the spatial extent of backwater influence, and the locations of our study sites, we find that the loss of mill dams explains the observed trends in bank erosion rates at 9 (and possibly 10) of our 14 monitoring sites. These results support the hypothesis that the demise of mill dams has been an important influence on fluvial processes in the region.