Skip to main content
King County logo
Sign up for announcements about this project

Project overview

South Fork Snoqualmie River corridor planning, funded by the King County Flood Control District (external link), set out to analyze flood and erosion risks to people and infrastructure and to develop a long-range strategy for reducing those risks.

The upper Snoqualmie Valley has experienced several destructive floods in the last 20 years. Flooding on the South Fork Snoqualmie River creates considerable public safety risks and impacts to both local and regional economies.

  • A 100-year flood event (a flood event that has a 1% probability of occurring in any given year) would likely inundate more than 130 structures and properties, over five miles of roadway including a portion of I-90, and critical facilities including North Bend’s wastewater treatment plant.

  • A 500-year flood event (a flood event that has a 0.2% probability of occurring in any given year) would create significantly more risk, inundating an estimated 500 structures, over 15 miles of roads, and additional critical facilities. 

The study area encompasses six miles of the South Fork Snoqualmie River from the confluence in Three Forks Natural Area to upstream of the I-90 bridge over the South Fork Snoqualmie River.

The planning effort involved:

  • Characterizing existing physical, biological, and social conditions in the corridor;
  • Identifying the location, nature, and significance of flood and erosion risks and ecological needs throughout the corridor;
  • Developing concepts for project and program actions to reduce vulnerability to flood risks and prevent or mitigate impacts to habitat or other valued uses in the river corridor; 
  • Developing a comprehensive and strategic approach to implementing capital improvement projects; and
  • Partnering with stakeholders at key milestones throughout the planning process.

The outcome is a Capital Investment Strategy proposing a sequence of coordinated projects to address the most critical flood and erosion related public safety risks.

Proposed actions are intended to:

  • Reduce vulnerability to flood risks for residents;
  • Protect public infrastructure;
  • Minimize disruption to the regional economy as a result of flooding;
  • Protect habitat and other community values; and
  • Partner with residents, community groups, and other resource agencies to share information and develop projects that provide multiple benefits.
  • Attached is the final South Fork Snoqualmie Capital Investment Strategy. It highlights the near-term, medium-term, and long-term actions that were identified in this planning process and that address a range of critical flood and erosion risks. The strategy was adopted as policy guidance by the King County Flood Control District's Executive Committee in July 2017. Also attached are the project location map and the implementation schedule for the capital investment strategy.

Public involvement


  • Oct. 15 - Project staff hosted a public meeting to provide the community with more information about flood risk reduction efforts being planned for the South Fork Snoqualmie River. Below are the agenda and presentation from that meeting. (Get Acrobat help.)



Project goals

The South Fork Snoqualmie River Corridor Plan is guided by four project goals established by the King County Flood Control District on December 16, 2014 (FCD Resolution FCD2014-21.1).

  • Reduce risks from flood and channel migration hazards - Plan for a 500-year desired level of flood protection based on the likely consequences of flooding to public safety, infrastructure, and uncertainty associated with climate change and sediment accumulation. Plans should also address identified geotechnical deficiencies and mitigate the potential for channel migration over the next 50-years.
  • Reduce the long-term costs of flood hazard management - Plan for short- and long-term cost effectiveness to reduce life-cycle floodplain management costs over time.
  • Improve the natural environment through sound and sustainable flood hazard management - Plan to improve instream and riparian habitat quality and quantity.
  • Incorporate stakeholder and community values into the corridor planning process - Plan for equitable considerations of diverse community and stakeholder interests and creation of actions that accomplish multiple objectives.

The South Fork Snoqualmie River Corridor Plan aims to identify and prioritize a series of actions that reduce risk and are ecologically and financially sustainable over time.

Process and timeline - updated July 2017

  • 2010-June 2014 - Characterize existing corridor conditions. COMPLETED. Read the report.
    • Flood, geotechnical, channel migration hazards and risk.
    • Aquatic and riparian habitat.
  • December 2014 - King County Flood Control District Resolution FCD2014-21.1. Approved South Fork Snoqualmie corridor planning framework. COMPLETED.
    • Floodplain management goals.
    • Objectives.
    • Conceptual approaches.
  • June 2014-June 2015 - Technical evaluation of corridor planning conceptual approaches. COMPLETED.
    • Levee setbacks.
    • Raise levees in place.
    • Continue existing management practices.
  • Summer 2015 - Community outreach.
  • Fall 2015 - Incorporate community input in planning process.
  • Winter 2016 - Plan approval and adoption.
  • Summer 2017 - Prepare final Capital Investment Strategy Report.
  • July 2017 - The South Fork Snoqualmie River Capital Investment Strategy was presented to the Flood Control District's Executive Committee. The strategy, which highlights near-term, medium-term, and long-term actions the County should take to address identified risks, now provides policy guidance and direction to the County.

Additional resources

(Get Acrobat help.)

For project questions contact Chrys Bertolotto, River and Floodplain Management, Department of Natural Resources and Parks.