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Project update

The King County Flood Control District (District) approved the Green River System-Wide Improvement Framework (SWIF) in March 2019 and it was accepted by the United States Army Corps of Engineers (Corps of Engineers) in September 2019.

The Green River SWIF outlines a prioritized strategy to address levee deficiencies in order to optimize flood risk reduction, address system-wide issues, and maintain eligibility for the 17 miles of levees currently enrolled in the Corps of Engineers’ Levee Rehabilitation and Inspection Program under Public Law 84-99 (PL 84-99).

The SWIF includes a suite of actions to address levee deficiencies and system-wide issues including:

  • Near term repairs focused on specific levee deficiencies;
  • Capital projects to address concerns such as slope stability and increase overall levee performance;
  • Programmatic activities to ensure effective operation and maintenance of flood facilities’ and
  • Interim risk reduction measures such as flood preparedness planning

Regional considerations related to capital project design, repairs, levee operation and maintenance, including the management of vegetation, are also included as part of the SWIF and supporting appendices.

Project documents

(For help viewing PDF files, see the King County Acrobat Help Page.)

Project background

The Green Duwamish River Watershed is home to salmonid species listed under the federal Endangered Species Act (ESA), valuable agricultural resource lands, and recreational trail networks. Regional transportation infrastructure, residential development, and commercial/industrial economic assets in the watershed also contribute to over one-eighth of Washington state’s annual Gross Domestic Product (GDP).

Within the Lower Green River, the District manages approximately 28 miles of levees and revetments, 17 miles of which are enrolled in the Corps of Engineer’s’ PL 84-99 program. The PL 84-99 program provides federal funding assistance for repair of levees damaged during a flood event. Levees enrolled in the PL 84-99 program must meet a set of standard inspection criteria to be eligible for funding assistance.

Many of the levees and revetments along the Lower Green River were originally constructed in the 1950s to only protect agricultural lands. Since that time, significant regional economic infrastructure, businesses and residential land uses have been implemented. The level of flood protection the existing levees and other flood facilities provides varies. And the levees often have over steepened side slopes that are subject to erosion. There were also many items identified through levee inspections that needed to be resolved. Additionally, studies by the Corps of Engineers in 2012 concluded that the Howard Hanson Dam, located near the headwaters of the Green River, can only control a 140-year flood event rather than the previously assumed 500-year level of protection.

Local jurisdictions throughout the Green River basin are responsible for:

  • implementing salmon recovery plans under the ESA,
  • complying with the Clean Water Act (CWA),
  • complying with the Federal Emergency Management Agency's (FEMA) development standards; and
  • mitigating impacts on habitat that may result from flood risk reduction projects. 

The Corps of Engineers established a SWIF policy in 2011 for non-federal levee sponsors to plan and implement levee improvement actions which may require a longer term comprehensive approach in order to address competing federal mandates and legal requirements that apply to levee systems and the riverine environment in which they're located.

In February 2013, the Corps of Engineers approved the District's Letter of Intent (2MB, PDF) to complete a SWIF for the Green River within a three-year timeframe to address levee inspection items and to plan for longer term levee improvements.

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

An Interim Green River SWIF (Interim SWIF) was approved by the District and submitted to the Corps of Engineers in February 2016 to meet the required timeline. The Corps of Engineers accepted the Interim SWIF in March 2017. As part of the SWIF, a provisional level of protection of 18,000 cubic feet per second (500-year flood) plus three feet of freeboard was approved for planned capital projects. The Interim SWIF focused on addressing levee inspection items and specified broader objectives, identified by stakeholders who participated as SWIF advisors, that would best be achieved through a long-range planning process that includes a State Environmental Policy Act (SEPA) environmental impact statement.

The Interim SWIF provided continued eligibility for levee rehabilitation assistance for two years and granted the District, as sponsor, the ability to finalize the SWIF and begin implementation of identified activities. For additional information, please visit the King County Flood Control District website.

Project timeline

Implementation of the SWIF is ongoing. The implementation period for the identified SWIF capital projects is anticipated to be 12 years. During the SWIF implementation period, levees will continue to be inspected and maintain eligibility in the PL 84-99 program so long as progress continues to address levee inspection items.

2012-2015 interagency collaboration and advisory process

The King County Flood Control District convened a collaborative interagency and advisory process to support preparation of the Interim SWIF. Many constituent interests contributed to the technical products that informed the Interim SWIF.

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

Council and committee members were engaged early and ongoing throughout the preparation of the Interim SWIF. Each group’s meetings were open to the public and organized to provide timely input and recommendations to the project in support of product development and District decision-making at critical project junctures. Project partners and advisors include representatives from the following organizations

  • King County Flood Control District
  • King County
  • Green River valley cities of Tukwila, Renton, Kent, and Auburn
  • Muckleshoot Indian Tribe
  • State agencies including: Puget Sound Partnership, the Department of Fish and Wildlife and the Department of Ecology
  • Federal agencies including: the U.S. Army Corps of Engineers, 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).

Development of the Lower Green River SWIF was guided by six project goals, as determined by the project’s advisory groups and consistent with the 2013 Letter of Intent. A motion to approve the SWIF vision and goals was approved by the King County Flood Control District Executive Committee on Jan. 27, 2014.

View the complete Green River SWIF project goals and their associated objectives (156KB, PDF).

2012-2015 Document Library


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.