Flood Control on the Sammamish River


Willowmoor, or the Transition Zone, refers to the area at the north end of Lake Sammamish where it drains into the Sammamish River.  You walk past Willowmoor/the Transition Zone as you leave the parking lot for the off-leash dog area at Marymoor Park.  The Transition Zone is an engineered channel that was constructed in the 1960s by the Army Corps of Engineers (the Corps) to provide downstream flood protection while maintaining specific lake levels.  It includes a weir (a small dam that is usually underwater) and a straight, relatively steep channel.  In fact, this is the steepest section of what is otherwise a flat and slow moving Sammamish River.

Where the Corps in the 1960s was concerned with maintaining minimum lake levels, the development that has occurred along the River and the Lake and all their tributaries, in addition to the increased frequency of big storms, means we are now concerned with how often the Lake level rises to the point of submerging docks and causing other property damage.

In response to concerns raised by Lake Sammamish property owners, the Flood Control District (FCD)* has been working on a project to increase the outflow from the Lake and moderate the peaks in lake levels.  The first step was to increase the maintenance done in the Transition Zone by clearing out willows and other obstructions on an annual basis.  This effort has notably reduced the frequency of flooding along the Lake, although it still occurs.  The other step is to reconfigure the Transition Zone to reduce the frequency and duration of high lake levels, and improve salmon habitat and recreational opportunities.

Since 2013, the FCD’s contractor, King County Water and Land Resources Division, has been collecting data, conducting technical analyses and developing a suite of conceptual design alternatives.  Simultaneously, the project team has engaged with various stakeholders and the public to seek input on alternatives for how to reconfigure the Transition Zone.  These alternatives include:

  1. Status Quo – continue current maintenance practices
  2. Widen the Existing Channel – Modify the existing channel for increased water flow out of the lake. Modify the existing weir for safe recreational navigation, lake level moderation and downstream flood protection.
  3. Split Channel – Modify the existing main channel and weir to optimize water flow and provide safe recreation navigation and construct e anew side channel as a primary salmon migration route.

Along with changes to the channel, the project is also exploring cold water supplementation ideas as a means to decrease the temperature of the water in the Transition Zone because salmon like cool water.

On June 20, 2016, the FCD Executive Committee voted to move ahead with the Split Channel option to 30% design.  The Executive Committee listed a number of specific issues to be explored during the next phase, including fully understanding the potential impact to downtown Redmond if more water flows through the Sammamish River, exploring the possibility of a dynamic weir that could be adjusted based on Lake levels, looking for funding for the cold water supplementation alternatives, and analyzing the impact to migrating salmon of the selected alternative.  The public and stakeholders will continued to be involved with the project.

To learn more about the project and monitor its progress, please visit the project website.


*  The Flood Control District is a separate government formed in 2007 that collects a countywide property tax to fund flood reduction projects on the rivers throughout King County.  The FCD is governed by a Board of Supervisors, which is composed of King County Councilmembers.  An Executive Committee, made up of five District Supervisors including Claudia Balducci, oversees the operations of the District.  The District contracts with King County’s Water and Land Resources Division for most of the projects it funds, along with the ongoing maintenance of levees and other flood facilities and functions.  You can learn more about the Flood Control District at its website.