Dam decommissioning is rapidly emerging as an important river restoration strategy in the United States. Hundreds of dams have been removed in the past few decades, including several large ones (>10–15 meters) impounding large sediment volumes (>106 cubic meters) in the past 3 years, notably Condit Dam and the Elwha River dams in Washington State. These removals and the associated studies provide for the first time an opportunity to evaluate the immediate and persistent consequences of these significant fluvial—and in some cases, coastal—perturbations. Understanding dam removal response not only improves understanding of landscape and ecosystem adjustment to profound sediment pulses but also provides important lessons for future watershed restoration efforts.
An effort to synthesize existing studies and data, thereby providing scientists and managers better knowledge of likely outcomes of future dam removals, is being sponsored by the U.S. Geological Survey’s (USGS) John Wesley Powell Center for Analysis and Synthesis. The 20-member working group convened the first of two meetings in June at the Powell Center facility in Fort Collins, Colo. The group consists of federal agency and academic biologists, hydrologists, geomorphologists, and engineers from throughout the United States.
The group discussed dam removal in the United States in the context of climate change and increasing human population, factors that because of their relation to water and food resources are at the forefront of international debates on how best to manage rivers. Discussions and presentations during this meeting revealed the tremendous growth of information from dam removals over the past few years and pointed to emerging patterns of physical and biological responses.
Reservoir size, degree of filling, and sediment characteristics, in conjunction with the type and rate of dam removal, appear to lead to predictable relations between processes eroding reservoir sediment and the downstream sediment transport and channel response, which in turn affect biological conditions and trajectories. For example, the distinctly different sedimentary and geomorphic responses to the instantaneous removal of Condit Dam on the White Salmon River compared to the recently completed phased removal of Glines Canyon Dam on the Elwha River highlight the interrelated roles of dam removal style, reservoir sediment characteristics, and the role of flow sequencing in transporting reservoir sediment downstream.
The scientific and management implications of such findings will be the focus of multiple analysis and reporting efforts over the next year as the group prepares for a final synthesis meeting in June 2015. These activities include building a database of dam removals and available data, analyses of biotic and abiotic process controls and linkages, geographic factors affecting dam removal and their consequences, common management concerns associated with dam removals, and insights relevant to these concerns gained from recent dam removals. These efforts will result in publications as well as an overall synopsis of current scientific knowledge of dam removal.
The working group principal investigators are Chauncey Anderson, Pat Connolly, Jeff Duda, Amy East, Jim O’Connor, and Powell Center postdoctoral fellow J. Ryan Bellmore. For more information on this specific Powell Center working group, contact Jim O’Connor ([email protected]); for more information on the general synthesis and analysis activities supported by the USGS Powell Center, visit https://powellcenter.usgs.gov/.
—Jim O’Connor, U.S. Geological Survey (USGS), Portland, Ore.; email: [email protected]; and Amy East, USGS, Santa Cruz, Calif.