Historical Shoreline Evolution as a Response to Dam Placement on the Elwha River, Washington

AbstractBethany M. NagidHistorical Shoreline Evolution as a Response to Dam Placement on the Elwha River, WashingtonMorphological changes of the Elwha River delta shoreline in Washington are analyzed from 1870-2015, revealing year-by-year as well as location-based geomorphological evolution. Change in the Elwha shoreline prior to the placement of two dams is shown as accretion in two of three areas of the delta with overall change of up to ~20-30m. To the east of the Elwha River mouth, annualized erosion rates during the lifetime of the dams averaged ~1m/year, but increased in recent years, regularly exceeding 4m/year between 2009 and dam removal in 2012. Other areas showed no significant trends during the overall time period of dam-use, but exhibit a wide variety of year-to year changes. Shoreline changes following dam removal (2012-2015) have shown a wide variety of responses: (1) an accreted shoreline beyond the extent of any previous year (west of the Elwha mouth); (2) an accreted shoreline not yet returned to the spatial extent prior to dam placement (east of the Elwha mouth), and (3) an eroded shoreline beyond previous shore configurations (east of Point Angeles). This survey of over 130 years of shoreline data at the Elwha delta is intended to identify differences in shoreline morphology during distinct periods of dam placement, use, and removal. The large-scale changes in shoreline morphology associated with dam placement and removal will be important processes to understand as traditionally dammed river ecosystems transition to dam removal in coming years