ABSTRACT
The strategy for removing a dam has been reported as the single most critical factor for managers influencing the outcome of removal[33] and the removal of dams may occur in a number of ways. Dams may be completely removed from the river or breached and partially removed, leaving bulkheads and sills in the river as structural artifacts. Dams may be removed instantaneously or dewatered and removed in stages, allowing the channel to erode reservoir sediments more slowly over time. The previous option (instantaneous removal) tends to be more common at small dam sites where rapid headcut erosion is less likely to initiate lateral erosion through gully walls. In contrast, larger dams tend to be slated for removal by staged breaching, allowing terraces to form in the stored sediment, stabilizing the material and preventing lateral erosion. Dams tend to be removed during periods of low flow when transport capacity limits the suspension of fine sediment.[17] To further reduce the rate and extent of erosion during removal, reservoirs may be drawn down gradually, sediment screens and traps may be installed, and stability measures may be taken.[34]
Whether in response to concern for downstream habitats or owing to a hazard posed by contaminants attached to reservoir sediments,[18,35] managers may choose to: (1) remove the sediment stored behind the dam, typically by dredging or by conventional excavation after reservoir drawdown; or (2) leave the sediment in place, with or without some structural stabilization of sediment stored behind the dam. Stabilization practices may include regrading, revegetating, or armoring exposed sediment to reduce erosion,[36,37]
or construction of grade control structures to fix the elevation of the riverbed. Owing to the cost of removal and structural stabilization, sediment is often left in place at small dam removals, allowing the river to
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erode through the reservoir sediments at a rate that depends on the hydrologic regime, sorting and volume of stored sediments, removal strategy, and width of the reservoir relative to the width of the channel.[32]
UNCERTAINTY IN THIS EMERGING SCIENCE
A great deal of uncertainty about the consequences of dam removal exists,[38,39] particularly related to the extent, magnitude, and timing of physical and ecological outcomes.[32,39] Confident predictions of ecosystem responses to dam removal are unrealistic,[40] and experience with and documentation of dam removal is limited.[32] Although approximately 450 dams have been removed in the last 100 years,[41] less than 5% of these removals resulted in published ecological research.[39] This work is both difficult to obtain and of limited value because of unsystematic data collection and lack of comprehensive reporting.[32,39] As a consequence of a lack of examples from which to derive expectations and insufficient documentation and analysis, the practice and science of dam removal are essentially new.
