King County rivers and floodplains provide important ecological, economic, and cultural benefits to local communities and tribes. Benefits include water quality improvement, flood storage and conveyance, habitat for threatened and endangered salmon, recreation, employment, and local food production. Fully functional floodplains allow the river to inundate and migrate in the areas needed to provide these natural, economic, and cultural benefits.
King County rivers can also pose significant flood and channel migration risks to people living and working in floodplains. River flooding is the County’s most frequent and costliest natural disaster, with extreme flooding occurring every two to five years on average.15 The Snoqualmie River is the most flood-prone watershed in King County.
While flooding and channel migration can produce negative consequences and threaten public safety and property, flooding and channel migration are both natural occurrences. In areas where floodplains and watercourses remain connected or have been reconnected to their floodplains, periodic floods help to create and maintain channel networks, floodplain wetlands, and vital and productive habitats.16 Preserving, protecting, and enhancing river floodplains in ways that reduce flood risks and benefit habitat can provide win-win solutions for reducing the impacts of climate change on river flooding.
What's at stake
Climate change increases the potential for river flooding and channel migration. While results will vary by location and by the return frequency of different flood events (e.g., the 1 percent annual chance flood event versus the 10 percent annual chance event), floods in the coming decades are likely to be larger and/or occur more frequently due to the combined effects of wetter winters, more intense heavy rain events, and more winter precipitation falling as rain rather than snow in mountain watersheds. The likelihood of increased flooding is greatest on unregulated rivers like the Snoqualmie but even rivers with dams, like the Green and White rivers, could see increased flooding.
Flooding can threaten livelihoods; damage homes, businesses, and built infrastructure; and disrupt critical services such as transportation, water supply, and agriculture. A recent flood risk assessment in King County estimated that a 1 percent annual chance river flood event could potentially cause county-wide damages equal to or exceeding $368 million.17 Low-income communities, renters, people with limited English proficiency, and BIPOC communities face greater challenges in recovery and resilience. Meanwhile, past efforts to reduce flood risk and manage floodplain land uses have harmed and reduced habitat essential to support salmon populations, affecting the ability of tribes to fully exercise their treaty rights.
A better outcome
King County envisions resilient communities and floodplains where flood risks to people and structures are reduced while supporting salmon recovery, agriculture, open space, and other community priorities. Floodplains are connected and restored, increasing their ability to naturally store and convey flood waters, protect water quality, sustain habitat, and build healthy soils for agriculture. Flood management infrastructure is planned and built to withstand climate change. Floodplain residents have the knowledge, resources, and capacity to protect themselves before and during floods, with equitable access to programs that reduce flood risk and support recovery.
What we've done to get here
- Included climate change as a major strategic priority in the 2024 update of the King County Flood Management Plan.
- Piloted an approach for incorporating climate change into capital planning for flood management projects as part of the Tolt River Levee Level of Service project, with funding from the Flood Control District and grants.
- Moved levees back to allow more space to store flood water and restore floodplain habitat, where feasible, with funding from the Flood Control District, and other local, state, and federal partners.
- Elevated 66 homes and acquired 215 at-risk properties and 600 acres since 2008, with funding from the Flood Control District.
- Partnered with the University of Washington (UW) Climate Impacts Group to initiate an assessment of climate change impacts of flooding on King County rivers, with funding from the Flood Control District.
15 King County Department of Natural Resources & Parks, “King County Flood Management Plan,” 2024.
16 King County Department of Natural Resources & Parks, “King County Flood Management Plan,” 2024.
17 King County Department of Natural Resources & Parks, “King County Flood Management Plan,” 2024.