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Willowmoor Floodplain Restoration Project video (5:49; external link).


In the mid-1960s, the U.S. Army Corps of Engineers constructed the Sammamish River Flood Control Project – a nearly 14-mile long project from Lake Sammamish to Lake Washington to provide flood protection to the Sammamish River valley. The project effectively lowered the bottom of the Sammamish River by more than six feet. The Transition Zone, a key component of this project, is essentially a ramp near the Lake Sammamish outlet that connects the old river channel to the deepened river channel downstream. This straight, rock-armored channel is approximately 200 feet wide and drops more than six feet along its 1,500 foot length. During low to moderate lake levels, lake outflow is contained within a 30-foot-wide, low-flow channel running down the center of the Transition Zone. Willow buffers, 10 feet deep, line both banks of this low-flow channel. During higher lake levels, lake outflow occupies the entire width of the Transition Zone.

At the upstream end of the Transition Zone, a concrete weir controls lake outflow, maintaining minimum summer lake levels. A narrow notch in the middle of the weir provides both up and downstream passage for Chinook salmon and other fish species and allows downstream passage for small boats such as canoes and kayaks. During high lake outflows, flow control shifts from the weir to the Transition Zone.

King County crews annually maintain the Transition Zone, including trimming the willow buffer and mowing the grass. Though currently required, these maintenance practices harm habitat and water quality and conflict with federal, state and local goals to protect ecological functions and species listed under the Endangered Species Act.

Overall, these efforts, known as the Sammamish River Flood Control Project, have been very successful in limiting flooding in the Sammamish River Valley. However, its construction and corresponding development have drastically impacted habitat conditions in the river channel and adjacent floodplains. In the vicinity of the Transition Zone, the poor physical habitat conditions are compounded during the summer by higher temperature water from the Lake Sammamish outflow that is harmful to salmon.

Project goals

The project proposes to reconfigure the Sammamish River Transition Zone and adjacent undeveloped King County property in a manner that meets the following three project goals:

  1. Ensure the Transition Zone’s capability to provide necessary lake level control, flow conveyance and downstream flood control.
  2. Enhance habitat conditions in the river channel, floodplain, buffers, associated tributaries and adjacent wetlands for ESA-listed Chinook, steelhead, and other fish and wildlife species.
  3. Reduce costs, complexity and ecological impacts of construction, operation and maintenance.

A suite of design objectives has been developed for each of the three goals along with supporting quantitative criteria. Additionally, several objectives outside of the three goals have also been developed, primarily associated with recreational opportunities and constraints.

Project area

The project proposes to reconfigure the Sammamish River Transition Zone, which is at River Mile 13.0-13.5 of the Sammamish River in Marymoor Park. The project area includes the river channel and adjacent undeveloped land to the southwest of the river channel.

Public involvement

  • Jan. 27, 2013 - First public meeting was held in Marymoor Park to kick off the Willowmoor Project. View the presentation (PDF, 2MB).
  • Aug. 2013 - King County convened a Stakeholder Advisory Committee (SAC) to provide input through selection of a preferred design alternative.
  • March 14, 2015 - A second public meeting was held March 14 at the Redmond Senior Center. 
  • Oct. 14, 2015 - Final Stakeholder Advisory Committee Meeting. All SAC meetings are open to the public.
  • June 6, 2016 - The King County Flood Control District held a special Executive Committee meeting in Bellevue to help inform the decision about which alternative will move forward for further design.

Accomplishments to date

The project began in early 2013. Accomplishments to date include:

  • hosted a project kick-off meeting, Jan. 27, 2013;
  • conducted several technical studies;
  • convened a Stakeholder Advisory Committee and conducted eight meetings;
  • developed a comprehensive suite of design objectives and corresponding quantitative criteria; and
  • produced multiple iterations of conceptual project design alternatives along with corresponding cost estimates.

Project timeline - Updated October 2017

Project Start May 2013
Stakeholder Advisory Committee Process Aug. 2013 – Apr. 2015 (est.)
Technical Studies
Jun. 2013 – Early 2016 (est.)
Design Objectives & Criteria Development
Oct. 2013 – May 2014
Draft Alternatives Development
Apr. 2014 – Apr. 2015
Preferred Alternative Selection and Approval
Oct. 2015 – Jul. 2016
Preliminary (30%) Design Jan. - Dec. 2018
Final Design and Permitting To be determined
Construction To be determined

Design alternatives development process

The project design team has developed a series of design alternatives to address the project goals. The identification, development, refinement and elimination of alternatives were based on their performance relative to the suite of design objectives, the results of various technical studies and guidance and feedback from the Stakeholder Advisory Committee.

The design alternatives were initially developed as two separate categories:

  • Channel reconfiguration – Alternative that would provide better physical habitat conditions while still maintaining essential flow conveyance through the Transition Zone.
  • Cold water supplementation – Alternatives that would supplement warm summer lake outflow with cold water to help lower temperatures to levels more suitable for migrating salmon.

During several iterations of review and refinement, some alternatives from each category were eliminated from consideration, while those determined to have the best potential were carried forward. Ultimately, the two most promising channel reconfiguration alternatives were combined with the two most promising cold water supplementation alternatives, resulting in two integrated alternatives.

These two integrated alternatives along with a “No Action” alternative (continue the ongoing maintenance) were reviewed to determine a preferred alternative tol be advanced for final design and construction.

Following are links to graphical depictions of each of these design alternatives (for help with PDF files, visit our Acrobat Help page):

The alternatives and their development process were discussed in greater detail in the Concept Design Summary Report, which was published in April 2015.

The Flood Control District reviewed these design alternatives and selected the "Split Flow Channel + Pumped Groundwater" alternative. The project team will hire a consultant to bring the project to 30% design. That work is expected to begin in January 2018.

Project documents

(For help viewing PDF files, see Acrobat Help page.)

For more information about the Willowmoor Floodplain Restoration Project, please contact Kate Akyuz, Project Manager, King County River and Floodplain Management Section.