Use of Seafloor Substrate Classification to Estimate Kelp Habitat Suitability Following Elwha Dam Removals

Abstract

Dam removals induce large scale disturbances to a system, having wide-spread impacts on the physical and biological characteristics of the rivers and coastal waters downstream. Removal of two large dams on the Elwha River, Washingtion has resulted in increases in sediment delivery to the Elwha delta affecting substrate composition and biological communities. Pre-dam removal monitoring has characterized substrate and community composition but uncertainties on the extent of deposition and resulting community responses to increased deposition remain. The relationship between acoustic backscatter of the seabed, seafloor substrate, and macrofaunal habitat was examined using sidescan sonar data and physical grab samples taken on the Elwha delta. Backscatter intensity analysis was combined with grain-size distribution allowing for classification of ‘suitable’ versus ‘probably suitable’ habitat. Habitat suitability pre and post dam removal was determined based on shifts in substrate on the subaqueous delta following dam removal. Acoustic backscatter results suggest an increase in fine-grain sediment, increasing the area of unsuitable habitat by 0.7 km2 since pre-dam removal conditions. Physical grab samples taken pre-dam removal reveal seafloor substrate was composed of primarily of gravel and sand, whereas post-dam removal conditions to date were characterized by sand and mud. Grain-size analysis from 2014 suggests a continuation of the shift toward fine-grained sediment. In the short-term it appears sediment supply in post-dam removal conditions is high enough to support the accumulation of mud on the delta, modifying the seafloor substrate and likely resulting in loss of kelp habitat. Whether these trends persist and kelp is permanently displaced remains unknown.