Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 7607
Title: Large-scale dam removal on the Elwha River, Washington, USA:  river channel and floodplain geomorphic change
Author: Amy East, G. R. Pess, Jennifer Bountry, Chris Magirl, Andrew Ritchie, Joshua Logan, Timothy Randle, Mark Mastin, J. J. Duda, Martin Liermann, Mike McHenry, T. J. Beechie
Publication Year: 2015
Journal: Geomorphology
Volume: 228
Pages: 765-786
Keywords: fluvial geomorphology,dams,dam removal,channel evolution,floodplain,sediment

 A substantial increase in fluvial sediment supply relative to transport capacity causes complex, large-magnitude changes in river and floodplain morphology downstream. Although sedimentary and geomorphic responses to sediment influx are a fundamental part of landscape evolution, few opportunities exist to quantify those processes over field scales. We investigated the downstream effects of sediment released during the largest dam removal in history, on the Elwha River, Washington, USA, having measured progressive changes in riverbed elevation and topography, bed-sediment grain size, and channel planform over two years of staged dam removal. Downstream translation and dispersion of a sediment wave, sourced from sediment in two former reservoirs, caused widespread bed aggradation of ~1 m (greater where pools filled), changed the river from pool-and-riffle to glide morphology, and decreased the slope of the lowermost river as approximately 940,000 t of new material accumulated in the mainstem and floodplain channels downstream from the dam sites. The new sediment deposition, which was finer than most of the pre-dam-removal bed, formed new bars and increased channel braiding, causing a transition from erosional- to aggradational-style channel avulsion. As a result of mainstem bed aggradation, floodplain channels received flow and filled with new sediment even during non-flood conditions. The river system showed a two- to ten-fold greater geomorphic response to dam removal than to a 40-year flood event four years before dam removal. Two years after dam removal began, the river had started to incise through sediment deposited as the initial sediment wave passed. Geomorphic alterations and changing bed-sediment grain size have important ecological implications, affecting habitat structure, benthic fauna, and salmonid fish spawning and rearing potential. The response of the Elwha River to dam removal represents a unique opportunity to observe and quantify fundamental geomorphic processes associated with a massive sediment influx, and also provides important lessons for future river-restoration endeavors. 



 This paper is part of a series of paper on the changes in sediment supply, transport, and deposition in the various environments of the Elwha River system due to the dam removal.

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Theme: Habitats to Support Sustainable Fisheries and Recovered Populations
Foci: Develop effective and efficient habitat restoration and conservation techniques.
Characterize the interaction of human use and habitat distribution, quantity and quality.