Impacts of the Elwha River Dam Removals on Chinook Salmon (Oncorhynchus tshawytscha) Spawning Habitat

Thesis (Master's)--University of Washington, 2021Streambed particle size is a key factor influencing Chinook salmon (Oncorhynchus tshawytscha) spawning site suitability. Both the construction and removal of dams alter the sediment in rivers, resulting in changes to suitable habitat for salmon. When in place, dams impede the movement of sediment, resulting in an accumulation of sediment above the dam and a degradation of the riverbed below the dam. The release of this accumulated sediment after dam removal can restore a more natural distribution of sediment size classes below the dam, potentially improving salmon spawning habitat. Dam removal for the purpose of river restoration and improving conditions for salmon has increased in recent decades. The purpose of this study is to assess the effects of the sediment released from two large dam removals on the Elwha River in Washington state on streambed particle size, and the resultant amount of suitable spawnable habitat for Chinook salmon. Pebble counts were conducted in 45 riffles in 2009, before dam removal, and in 21 riffles in 2015, after dam removal, in the mainstem of the Lower and Middle sections of the Elwha River. The fraction of movable particles by a spawning Chinook salmon in the riffle crests increased by 43% after dam removal, resulting in more suitable spawning habitat. After dam removal, there was greater variation in sediment size within riffle crests, whereas there were predominantly larger particles before dam removal. The positive changes in sediment measured in this project support the concept that dam removal is a meaningful strategy for salmon restoration