The Influence of Dams on Streams of the Mid-Atlantic Region, USA

Dams represent a significant perturbation on streams by influencing the supply of water and sediment to downstream reaches. We assessed the downstream effects of dams at fifteen sites of varying dam size in Pennsylvania and Maryland by comparing downstream reaches with an upstream control reach. We found that the bed material in the downstream reach is coarser than upstream. Specifically, downstream reaches have less mud, sand, and granules than upstream reaches, and more pebbles and cobbles. The fraction of boulders and exposed bedrock is statistically similar both upstream and downstream. Using a simple numerical model for the grain size distribution of bedrock influenced channels, we estimated that trap efficiencies for bed material are surprisingly high at our sites, ranging from around 10 percent to around 50 percent. We also found that dams had no consistent influence on channel width and slope, suggesting that dams do not significantly affect channel morphology at our sites. We attribute this to: 1) pervasive bedrock influence on streams in this region, 2) low regional sediment supply, so upstream reaches unaffected by dams are sediment starved, similar to downstream reaches and thus little sediment is available below dams to affect channel morphological change through deposition, 3) highly vegetated and cohesive banks that are difficult to erode, limiting possible width adjustment. Our results emphasize that geologic history and setting are primary factors controlling how dams influence stream channels.