Dam removal is an increasingly adopted approach to river restoration, particularly in the U.S. Many removed dams have been small, generating only minor and localized geomorphic effects, but several large-dam removals in recent decades have had more profound geomorphic consequences. Understanding the full spectrum of consequences is important, particularly since dam removal is a significant and purposeful river manipulation, sometimes with uncertain restoration outcomes and potential for human and ecological disturbances. As a group collaborating in synthesis studies conducted under the auspices of the USGS John Wesley Powell Center for Analysis and Synthesis, we evaluated studies of the physical responses of dam removal, assessing common primary controls as well as lingering uncertainties. This work draws from a recent bibliographic database of dam-removal studies (Bellmore et al., 2017, Wiley Interdiscip. Rev. Water, doi: 10.1002/wat2.1165) and is summarized more completely in a review of geomorphic effects of U.S. dam removals (Major et al., 2017, in Gravel-Bed Rivers: Processes and Disasters).
Only a few dozen studies detail physical responses to removals; nevertheless, we see some commonalities: (1) Rivers respond quickly to dam removals, especially when removals are sudden rather than prolonged or staged and the impounded sediment is coarse grained, such as sand and gravel. In such settings, rivers swiftly evacuate large fractions of reservoir sediment, as much as half in the year following removal. Channels downstream typically stabilize within months to years—not decades. (2) Modest streamflows (< 2-year return interval flows) can produce much of the initial geomorphic response, mainly because of steep and energetic conditions created by the local base-level fall of dam removal. (3) Dam height, sediment volume, and sediment caliber influence the post-removal reservoir landscape and downstream response to dam removal. Removals of tall dams with large volumes of impounded sediment have had longer-lasting and broader effects than the more common removals of small dams. (4) Downstream valley morphology, extent of human modifications, and position of a dam within a watershed influence the transport and distribution of released sediment. Not yet well known are the geomorphic consequences of flow changes owing to dam removal, mainly because very few removals so far have significantly affected flow.