Can Dam Removal Restore Threatened Shorelines? The Elwha Case Study

Abstract

Dam construction on rivers like the Elwha River in Washington State reduces sediment supply to adjacent coastal systems and has often been linked to coastal erosion and threats to human infrastructure. With the removal of two dams on the Elwha River, it was hypothesized that chronic erosion of the surrounding coastline would slow or reverse. To test the impact of dam placement and removal on shoreline change, shoreline change was quantified by digitizing shoreline position using a 1908 survey map and four aerial photographs spanning 1939-2014. Anthropogenic and natural features related to shoreline change were identified using a combination of aerial photographs, remotely-sensed data and field surveys. Shoreline change analysis revealed retreat of the vegetation line east of the river mouth both before and during dam removal and areas of net accretion to the west. However, for ~1 km east of the river mouth during dam removal, new sediment accreted along the shoreline where vegetation retreated. Although this indicates that in some places movement of the vegetation line is not a reliable proxy for shoreline change, it also reveals that dam removal has not slowed erosion in all areas of the eastern coastline. Local wave regimes and the locations of abandoned river channels, levees, and shoreline armoring correlate spatially with calculated patterns of shoreline change. This suggests the importance of considering all of the potential geologic and anthropogenic influences when predicting shoreline responses to major disturbances such as dam removal.