A Riverscape Approach to Assess Fish and Habitat Relationships Prior to Dam Removal on the Elwha River, Washington

Dam removal has been increasingly proposed as a river restoration technique, with several projects scheduled to occur in the western United States.  In 2011, two large hydroelectric dams will be removed from the Elwha River on Washington State’s Olympic Peninsula in one of the nation’s largest dam removal projects.  Ten anadromous fish populations are expected to recolonize ~130 km of historical habitats after dam removal.  A key to understanding watershed recolonization and ecosystem restoration is the collection of spatially continuous information on fish and aquatic habitats.  To date, no studies have described spatially continuous fish and habitat relationships prior to dam removal, and consecutive-year studies throughout an entire river are rare.  We conducted concurrent snorkel and habitat surveys in the Elwha River from the headwaters to the mouth (rkm 65 to 0) in 2007 and 2008.  This “riverscape” approach was used to characterize spatial extent, assemblage structure, abundances, densities, and length classes of Pacific salmonids along a nearly continuous longitudinal gradient of 316 channel units.  The longitudinal fish assemblage patterns revealed that species richness was highest below the dams, where anadromous salmonids still have access. The percent composition of salmonids was nearly identical in 2007 and 2008 for rainbow and cutthroat trout (89%; 88%), Chinook salmon (8%; 9%), and bull trout (3% in both years). Pink salmon were observed (<1%) in 2007 only.  Spatial patterns of abundance for rainbow and cutthroat trout (Pearson’s correlation, r = 0.76) and bull trout (r = 0.70) were consistent between years despite differences in river flows in 2007 and 2008.  Both multivariate and univariate analyses revealed clear differences in habitat structure along the river profile, due to both natural and anthropogenic factors.  The generated fish and habitat profiles helped to visualize fish and habitat relationships and revealed unexpected spatial variations in fish abundances.  This comprehensive view helped to highlight species-specific biological hotspots, revealing that 60-69% of federally threatened bull trout occurred near or below the dams.  The riverscape approach also helped to focus future monitoring efforts, and addressed linkages between fish and aquatic habitats prior to dam removal.  Spatially continuous surveys will be vital in evaluating the effectiveness of upcoming dam removal projects at restoring anadromous salmonids.  These surveys are part of a larger effort to complete an atlas of riverscapes in major Olympic Peninsula rivers.