Flooding in the Green-Duwamish River Watershed

The Green-Duwamish River watershed is entirely within King County. Auburn, Kent, Renton, Tukwila, Seattle, and unincorporated areas of King County are within the watershed. The land includes protected forests, agricultural lands, residential communities, and commercial areas. Public facilities and important roads, highways, and rail lines are also located in the watershed. Many people commute to and through the Green River Valley. The watershed is usually divided into four sub-areas:

• Upper Green River, which extends westward from Stampede Pass in the Cascades and includes the Howard A. Hanson Dam and reservoir. The Dam, which has been in operation since 1962, helps reduce the risk of flooding in the watershed. There is very limited access to this forested area of the watershed.
• Middle Green River, which begins downstream of the Howard A. Hanson Dam and runs to Auburn. The river meanders in this area. Much of this land is used for farming, with some residential development. There is important habitat in the Middle Green for fish and wildlife.
• Lower Green River, which runs through the cities of Auburn, Kent, Tukwila, and Renton to the Duwamish River near I-405. In this area, the river is more confined by development. The land around the river is home to many people and businesses, and even factories. There are also important agricultural areas.
• Duwamish River, which flows into Elliot Bay. This is another area with homes and businesses. There are also marinas and facilities to support commerce and transportation around the mouth of the river.

Flood risks in the watershed

• Flooding is common and widespread through agricultural and undeveloped areas in the Middle and Lower Green River.
• Areas in Auburn and the Middle Green are at risk of channel migration. This is a natural process where rivers can change paths over time.
• The Lower Green-Duwamish River is at risk of flooding in residential, commercial, and industrial areas, particularly in the cities of Auburn, Kent, Renton, and Tukwila.
• Tidal flooding from rising and lowering sea levels is a concern along the Duwamish River. Scientists expect the potential for this hazard to increase as sea levels rise.

King County, as service provider to the King County Flood Control District, monitors and maintains 28 miles of levees and revetments throughout the watershed. A levee is a raised structure built along a riverbank to contain, control, or divert the flow of water and protect land along the river from temporary flooding. A revetment is a structure built along a riverbank to prevent erosion. King County coordinates with the U.S. Army Corps of Engineers that owns and operates Howard A. Hanson Dam. Although it is not possible to prevent all flooding, the Dam and the levee system help reduce flood risks to neighbors and the community.

The importance of healthy floodplains

Connected floodplains provide space to store flood water and habitat for fish and wildlife. The Green-Duwamish River watershed provides important habitat for Chinook salmon and steelhead trout listed under the Endangered Species Act. 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.