Abstract

The Snohomish and Stillaguamish rivers are home to some of Puget Sound’s most valuable natural, economic, and cultural resources. From iconic salmon runs to a rich agricultural economy, these floodplains are vital to the long-term health and well-being of diverse communities, ecosystems, and species living within these watersheds. Climate change is a particular threat to floodplains because of the impacts on flooding, water availability, and a host of other key features of these areas.

In the Snohomish and Stillaguamish estuaries, continued sea level rise will increase the extent, depth and duration of coastal flooding and accelerate erosion along the shoreline. In the Lower Skykomish and Stillaguamish Valley, heavier rainstorms, declining snowpack, and increased winter streamflow will lead to larger, more frequent flood events. Beyond winter flooding, other climate impacts such as the increased frequency of extreme heat events may alter crop yields, while higher surface water temperatures threaten salmon recovery. Although flooding is not new for these communities, climate change will be experienced as a threat multiplier, exacerbating other pressures such as shifting land use, population growth, and heightened demand for development.

The purpose of this work is to support the effective integration of climate change into existing Integrated Floodplain Management (IFM) efforts in Snohomish County, specifically through the
identification of climate-relevant adaptation needs. The intent of this study is to produce an actionable adaptation agenda, by engaging local floodplain actors throughout the research process and prioritizing existing opportunities to mainstream climate adaptation into ongoing projects, plans, and policies.