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Project update

On Feb. 16, 2016, the King County Flood Control District approved the Green River System-Wide Improvement Framework Interim Report. On Feb. 22, 2016 the Interim SWIF was submitted to the U. S. Army Corps of Engineers for review.

The Interim SWIF Report allows levees along the lower Green River that are enrolled in the federal rehabilitation assistance program (PL 84-99) to remain eligible for repairs. The report is composed of six chapters, including an introduction, deficiency action plan, capital plan, vegetation plan, interim risk reduction measures, and funding and implementation. The Green River SWIF Advisory Council and Technical Advisory Committee, convened by the Flood District, helped to inform preparation of the Interim SWIF Report.

The project documents are available in Acrobat format.

Project overview

In February 2013, the U.S. Army Corps of Engineers (Corps, external link) approved the King County Flood Control District's (Flood District, external linkLetter of Intent (2MB, PDF) to complete a System-Wide Improvement Framework (SWIF) for the Green River by February 2015.

On March 21, 2014 the Corps issued a PL84-99 Interim Guidance and communicated the programmatic effects in correspondence to tribes and program sponsors. King County is committed to completing the Green River SWIF and is currently working with the Corps to understand how the Interim Guidance will affect the project’s scope and timeline.

A SWIF is a Corps-sanctioned process to achieve flood protection solutions that satisfy the multiple and often competing federal mandates and legal requirements that apply to levee systems, enrolled in the Corps' PL-84-99 program (external link) and the riverine environment in which they're located.

Photo of the Green River valley

The Green Duwamish River watershed is home to salmonid species listed under the federal Endangered Species Act (ESA), valuable agricultural resource lands, regional transportation infrastructure, recreational trail networks, and commercial/industrial economic assets that contribute to over one-eighth of Washington state’s annual Gross Domestic Product (GDP). The Lower Green River has been highly engineered over time yet it is still flood prone. The Flood District manages approximately 18 miles of levees along the Lower Green River, 16 miles of which are currently enrolled in the Corps' PL-84-99 program (external link).

The Green River SWIF will produce a prioritized set of capital project and programmatic recommendations to achieve reach-specific, Lower Green River flood protection goals in a manner that builds economic, ecologic and community resiliency for current and future generations.

Geographic scope

The Green River SWIF project area includes the upstream extent of the Howard Hanson Dam at river mile (RM) 64.5 downstream to RM 5.5, the location of the farthest downstream Lower Green River flood protection facility managed by the Flood District. The focal point for the Green SWIF is the Lower Green River portion of the watershed, from RM 32 to RM 11. View the project map (1.5MB, PDF)

Why is the Green River SWIF needed?

The levee system in the Green River is old and outdated and originally constructed to protect agricultural lands rather than the significant regional economic infrastructure, businesses and residential land uses that exist currently. Additionally, the level of protection from flooding provided by current levees and other flood protection structures varies.

Recent studies (Assembly of Design Flood Hydrographs for the Green River Basin, U.S. Army Corps of Engineers, 2012, 1MB, PDF) by the Corps have concluded that the Howard Hanson Dam can only control a 140-year flood event rather than the previously assumed 500-year level of protection. There is a need to agree on a desired level of protection for the Green River, and determine how this protection can be provided by a combination of levee, and possibly dam, improvements.

Local jurisdictions throughout the Green River basin are responsible for implementing salmon recovery plans under the ESA, complying with the Clean Water Act (CWA), the Federal Emergency Management Agency's (FEMA) development standards, and mitigating impacts on habitat that may result from flood risk reduction projects. These complex issues require key policy considerations of levee system management options and implications.

Project goals

Preparation of the Green River SWIF will be guided by six project goals, as discussed by the project’s advisory groups. A motion to approve the SWIF vision and goals was approved by the Flood Control District Executive Committee on Jan. 27, 2014. View the complete Green River SWIF project goals and their associated objectives (156KB, PDF).
  • Integrated river and floodplain management: Reach agreement on an integrated list of multi-objective, prioritized projects and non-regulatory, programmatic actions that achieve the Green River SWIF’s agreed to goals for level of protection from flooding. This integrated set of flood protection strategies and actions shall:
    • improve water temperature;
    • advance progress towards meeting salmon protection and recovery goals;
    • enhance open space, recreation, treaty fishing, and public access;
    • support farmland protection, resiliency and productivity; and
    • reduce long-term facility maintenance costs.
  • Flood protection: Define a desired level of protection based on the likely consequences of flooding to public safety, infrastructure, current and future economic development, and estimated costs of capital projects, maintenance, lost ecosystem services, and costs and duration of disaster recovery in the event of levee overtopping or failure.
  • Vegetation management: Develop shoreline and levee vegetation management recommendations to further the goals of the ESA, CWA, and Corps PL84-99 standards.
  • Ecological resiliency: Improve the ecological resiliency of the Lower Green River’s aquatic and terrestrial habitats through implementation of the Green River SWIF’s priority projects and non-regulatory, programmatic recommendations.
  • Economic resiliency: Improve the resiliency of the economic base of the Lower Green River Valley through implementation of the Green River SWIF’s priority projects and non-regulatory, programmatic recommendations.
  • Community resiliency: Improve the resiliency of communities in the vicinity of the Lower Green River through implementation of the Green River SWIF’s priority projects and non-regulatory, programmatic recommendations.

Project advisors and partners

The 2012 Green River SWIF Letter of Intent (2MB, PDF) to the U.S. Army Corps of Engineers described a proposed approach to completing a SWIF for the Lower Green River that provided for “diverse and timely input” to the development of the Green River SWIF. It further stated that the project’s advisors would be comprised of local, state and federal agencies, Native American Tribes, business and environmental interests, and resource agencies.

The Flood District has convened two advisory groups to inform the preparation of the Green River SWIF:

Council and committee members will be engaged early and ongoing throughout the preparation of the SWIF. Each group’s meetings are open to the public and organized to provide timely input and recommendations to the project, in support of product development and Flood District decision-making at critical project junctures. Project partners and advisors include representatives from the following organizations (external links):
  • King County Flood Control District
  • King County
  • Green River valley cities of City of Tukwila, City of Renton, City of Kent, and the City of Auburn
  • Muckleshoot Indian Tribe
  • State agencies, including Puget Sound Partnership, the Department of Fish and Wildlife and the Department of Ecology
  • Federal agencies, including U.S. Army Corps of Engineers and National Oceanic and Atmospheric Administration
  • Business community, including Boeing, Washington REALTORS, Master Builders Association, and NAIOP Commercial Real Estate Development Association
  • Environmental groups, including The Nature Conservancy and American Rivers

For more information regarding the SWIF’s advisory process, please see the Green River SWIF Advisory Charter and Work Plan (231KB, PDF).


The Green River SWIF is funded by the King County Flood Control District and a $300,000 grant from the State of Washington through Puget Sound Partnership (external link).

King County Flood Control District Logo

For more information, please contact Lorin Reinelt, Managing Engineer, River and Floodplain Management Section.