Evaluating temperature buffering capacity of floodplain side channels to promote refugia for salmonids in the Stillaguamish River basin

Abstract

In the Stillaguamish River basin, low streamflow and high stream temperatures during summer have been identified as barriers to Chinook (Oncorhynchus tshawytscha) and coho (Oncorhynchus kisutch) salmon recovery and will likely be exacerbated with climate change. Projections forecast higher stream temperatures, drier summers, more intense rain events, and less snowpack starting within the next 20 years. Groundwater seepage zones from side channels and tributaries are cooler than mainstems and can create cold-water refugia for spawning and rearing salmonids. Increasing groundwater storage potential of floodplains and the extent of side channel habitat through floodplain reconnection may buffer salmonids’ vulnerability to high stream temperatures. A watershed characterization study done by Snohomish County between 2010 and 2015 determined that restoration of side channels and riparian areas along mainstem habitat would provide temperature benefits to mitigate climate change (Leonetti 2015). This project builds upon the Stillaguamish Tribe of Indian’s knowledge and collaborations with Stillaguamish Watershed Council to identify variations in stream temperature, dissolved oxygen, stream type, cover type, and salmonid use of mainstem, side channel, and tributary habitats between five areas and 13 reaches within the Stillaguamish River basin. To help influence the Stillaguamish Tribe and Watershed Council's restoration planning efforts we conducted snorkel surveys in mainstem, side channel, and tributary habitats during August and September of 2018 to perform a reach-scale analysis. Results from a Pearson’s correlation test indicate air temperature and maximum stream temperature have a strong, positive correlation within our study reaches. In some mainstem reaches, stream temperatures will exceed salmonid thresholds (greater than 20 degrees Celsius) during the summer, but side channel and tributary inputs can create refugia within salmonid thresholds (10 to 17 degrees Celsius). Dissolved oxygen concentrations ranged from 6 to 12 milligrams per liter in most habitats but dropped to about 3.0 milligrams per liter in some disconnected side channel habitat. We observed the highest counts of salmonids when wood (natural or man-made) and vegetation were present, which confirms previous research that salmonids aggregate near these cover types. We performed quasi-Poisson regression analyses to explain the relationship we observed between fish counts for adult Chinook and juvenile coho and potential predictors of fish count. Restoration strategies, such as engineered logjams (ELJs), designed to reconnect floodplain side channels will likely increase the extent of cold-water refugia, potentially mitigating climate change impacts. We created reach-scale maps to illustrate stream temperatures and dissolved oxygen concentrations and developed a model to explain our salmonid counts. In doing so, this research supports floodplain restoration planning efforts and documents pre-restoration conditions to support future adaptive management throughout the Stillaguamish River basin.