Flooding in the Sammamish watershed

The main sources of the Sammamish River include Lake Sammamish and the Bear, Little Bear, North, and Swamp creeks. Much of the wide floodplain area in the middle and upper portions of the river is in unincorporated King County and protected for farming by the Farmland Preservation Program.

The Issaquah, Tibbetts, and Laughing Jacobs creeks all flow into Lake Sammamish.

Flood risks in the watershed

• Issaquah Creek flows through the City of Issaquah and creates an urban flood hazard area there.
• Flooding of recreational infrastructure and landscaping occurs regularly around Lake Sammamish. Find more information at Lake Sammamish flooding frequently asked questions (FAQs).
• In Redmond, Sammamish River flooding can impact the accessibility of Northeast 95th Street and low areas on the Sammamish River Trail. Flooding can also have a severe impact on stormwater and water quality infrastructure, at times backwatering into facilities.
• In the Agricultural Production District between Redmond and Woodinville, farmland flooding can occur when the river elevation is high and upland stormwater cannot drain effectively.
• From Woodinville downstream to Lake Washington, the river generally stays within its banks.

Flood infrastructure in the watershed

The entire Sammamish River is part of a flood control project completed by the U.S. Army Corps of Engineers in 1966. The project straightened the previously meandering river channel. King County was a partner and accepted responsibility for long-term maintenance of the project. The project was designed to provide agricultural crop protection after March 1.

The project includes a transition zone that connects Lake Sammamish to the Sammamish River. It is designed to control spring and summer outflows from the lake. The transition zone includes a low concrete dam, called a weir, that raises the level of water upstream. Immediately downstream of the weir is a steep 1,400-foot section armored with rock that provides the hydraulic control for the lake at higher flows. Every summer, King County, as a service provider to the King County Flood Control District performs annual maintenance of vegetation in the transition zone, to ensure adequate conveyance.

King County, as service provider to the King County Flood Control District, monitors and maintains revetments along Issaquah Creek and two of its tributaries: Fifteen-Mile Creek and Holder Creek. A revetment is a structure built along a riverbank to prevent erosion.

The importance of healthy floodplains

Kokanee salmon, which were once abundant in Lake Sammamish, are nearly gone due to land use changes and their impact on fish habitat. Issaquah Creek is one of the highest priority habitats for Chinook salmon protection and restoration. Carey Creek and Holder Creek, which flow into Issaquah Creek, also provide excellent salmon habitat.

Changing the natural or built environment can affect fish habitat, including water quality and temperature. We consider potential impacts to these species when designing flood risk reduction projects.