The Challenges of Dam Removal and River Restoration

River restoration is a societal goal in the United States. This collection of 14 research papers focuses on our current understanding of the impacts of removing dams and the role of dam removal in the larger context of river restoration. The chapters are grouped by topic: (1) assessment of existing dams, strategies to determine impounded legacy sediments, and evaluating whether or not to remove the dams; (2) case studies of the hydrologic, sediment, and ecosystem impacts of recent dam removals; (3) assessment of river restoration by modifying flows or removing dams; and (4) the concept of river restoration in the context of historic changes in river systems.
The rise and fall of Mid-Atlantic streams: Millpond sedimentation, milldam breaching, channel incision, and stream bank erosion
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Published:January 01, 2013
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CiteCitation
Dorothy Merritts, Robert Walter, Michael Rahnis, Scott Cox, Jeffrey Hartranft, Chris Scheid, Noel Potter, Matthew Jenschke, Austin Reed, Derek Matuszewski, Laura Kratz, Lauren Manion, Andrea Shilling, Katherine Datin, 2013. "The rise and fall of Mid-Atlantic streams: Millpond sedimentation, milldam breaching, channel incision, and stream bank erosion", The Challenges of Dam Removal and River Restoration, Jerome V. De Graff, James E. Evans
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Abstract
For safety and environmental reasons, removal of aging dams is an increasingly common practice, but it also can lead to channel incision, bank erosion, and increased sediment loads downstream. The morphological and sedimentological effects of dam removal are not well understood, and few studies have tracked a reservoir for more than a year or two after dam breaching. Breaching and removal of obsolete milldams over the last century have caused widespread channel entrenchment and stream bank erosion in the Mid-Atlantic region, even along un-urbanized, forested stream reaches. We document here that rates of stream bank erosion in breached millponds remain relatively high for at least several decades after dam breaching. Cohesive, fine-grained banks remain near vertical and retreat laterally across the coarse-grained pre- reservoir substrate, leading to an increased channel width-to-depth ratio for high-stage flow in the stream corridor with time. Bank erosion rates in breached reservoirs decelerate with time, similar to recent observations of sediment flushing after the Marmot Dam removal in Oregon. Whereas mass movement plays an important role in bank failure, particularly immediately after dam breaching, we find that freeze-thaw processes play a major role in bank retreat during winter months for decades after dam removal. The implication of these findings is that this newly recognized source of sediment stored behind breached historic dams is sufficient to account for much of the high loads of fine-grained sediment carried in suspension in Mid-Atlantic Piedmont streams and contributed to the Chesapeake Bay.
- Appalachians
- cohesive materials
- dams
- erosion
- hydraulics
- incised valleys
- Maryland
- mathematical models
- North America
- Oregon
- Pennsylvania
- Piedmont
- quantitative geomorphology
- reclamation
- river banks
- safety
- sedimentation
- site exploration
- tailings dams
- tailings ponds
- United States
- Little Falls
- Mountain Creek
- Marmot Dam
- Hammer Creek
- Eaton-Dikeman Reservoir
- Canoy Creek