Predicting floodplain and fish community responses to the removal of the Elwha River dams, Washington State, USA

The Elwha River dams have disconnected the upper and lower Elwha watershed for over 90 years, disrupting upstream salmonid migration to 90% of the salmonid habitat. The dams have also interrupted the downstream movement of both sediment and wood, leading to such inputs being dominated by local sources. Current salmonid populations are either dramatically smaller than estimated historical population or extirpated. Nevertheless, salmonid populations do persist below the dams in part because channel incision has not been significant, and floodplain habitats remain an important component of the Elwha River ecosystem. Impending removal of these dams presents an opportunity to explore linkages among changes in salmonid populations, sediment supply, in-channel wood abundance, and habitat and ecosystem attributes. Dam removal will (1) reconnect upstream habitats increasing salmonid carrying capacity, and (2) allow the downstream movement of sediment and wood leading to long-term aquatic habitat improvements. Both large-scale changes will allow salmonid populations to rebuild on a watershed-scale. To understand how these changes alter the spatial and temporal dynamics of the river-floodplain ecosystem, we combine data for erosion and formation of floodplain surfaces, successional patterns of floodplain vegetation, and responses of riverine fauna to the shifting suite of habitat types. Sampling of ecosystem attributes before and after dam removal, as well as in nearby reference rivers will elucidate functional relationships among salmonid populations, sediment and wood supply, formation and persistence of river and floodplain habitats, and resultant ecosystem dynamics.