Fish Presence/Absence and Stream Habitat in Areas Affected By Sediment from Mount Saint Helens Eruption

The 1980 eruption of Mount Saint Helens deposited over 3 billion cubic yards of sediment, volcanic ash and other debris in the upper North Fork Toutle River (NTFR) valley. To mitigate the impacts of continual debris loading in the NTFR valley, the U.S. Army Corps of Engineers constructed a sediment retention structure (SRS) soon after the eruption to prevent excessive sediment loads from entering the Toutle, Cowlitz, and Columbia River systems. Construction of the SRS blocked access to historic spawning grounds of coho salmon (Oncorhynchus kisutch) and winter steelhead (O. mykiss).  A fish collection facility was constructed below the SRS to capture and transport these returning adult fish to Alder and Bear creeks, both tributaries of the upper NFTR.   The Pacific Northwest National Laboratories evaluated spawning success and rearing in the tributaries during August 2013 through July 2014.  Additionally, stream habitat and the migration corridors were evaluated.  The current practice of capturing and transporting adult salmonids above the SRS appears to be successful in terms of production of juvenile offspring in the Alder and Bear/Hoffstadt drainages; however, uncertainty remains regarding the overwinter rearing and outmigration success of the juvenile salmonids in these systems.