Paper No. JAWRA-07-0006-P of the Journal of the American Water Resources Association (JAWRA). Discussions are open until June 1, 2009.
Influence of Small Dams on Downstream Channel Characteristics in Pennsylvania and Maryland: Implications for the Long-Term Geomorphic Effects of Dam Removal1
Version of Record online: 29 OCT 2008
© 2008 American Water Resources Association
JAWRA Journal of the American Water Resources Association
Volume 45, Issue 1, pages 97–109, February 2009
How to Cite
Skalak, K., Pizzuto, J. and Hart, D. D. (2009), Influence of Small Dams on Downstream Channel Characteristics in Pennsylvania and Maryland: Implications for the Long-Term Geomorphic Effects of Dam Removal. JAWRA Journal of the American Water Resources Association, 45: 97–109. doi: 10.1111/j.1752-1688.2008.00263.x
- Issue online: 27 JAN 2009
- Version of Record online: 29 OCT 2008
- Received January 12, 2007; accepted June 10, 2008.
- fluvial processes;
- hydraulic structures;
- sediment transport;
- environmental impacts
Abstract: We evaluate the effects of small dams (11 of 15 sites less than 4 m high) on downstream channels at 15 sites in Maryland and Pennsylvania by using a reach upstream of the reservoir at each site to represent the downstream reach before dam construction. A semi-quantitative geomorphic characterization demonstrates that upstream reaches occupy similar geomorphic settings as downstream reaches. Survey data indicate that dams have had no measurable influence on the water surface slope, width, and the percentages of exposed bedrock or boulders on the streambed. The median grain diameter (D50) is increased slightly by dam construction, but D50 remains within the pebble size class. The percentage of sand and silt and clay on the bed averages about 35% before dam construction, but typically decreases to around 20% after dam construction. The presence of the dam has therefore only influenced the fraction of finer-grained sediment on the bed, and has not caused other measurable changes in fluvial morphology. The absence of measurable geomorphic change from dam impacts is explicable given the extent of geologic control at these study sites. We speculate that potential changes that could have been induced by dam construction have been resisted by inerodible bedrock, relatively immobile boulders, well-vegetated and cohesive banks, and low rates of bed material supply and transport. If the dams of our study are removed, we argue that long-term changes (those that remain after a period of transient adjustment) will be limited to increases in the percentage of sand and silt and clay on the bed. Thus, dam removal in streams similar to those of our study area should not result in significant long-term geomorphic changes.