WRIA 9 Duwamish-Green Watershed

Duwamish River flowing under a foot bridge in the Duwamish-Green watershed.

Photo by John Gateley

This is a collection of water availability information for the Duwamish-Green Watershed.

The Duwamish-Green Watershed or Water Resource Inventory Area (WRIA) 9 is defined as the area that drains to the Duwamish-Green River. It is located in the southern Puget Sound area, and comprises most of southern King County, including south Seattle and its adjacent suburban areas of Kent, Des Moines, Auburn, and Covington.

WRIA 9 Duwamish-Green watershed

The Duwamish-Green Watershed has one major river, with the Green River becoming the Duwamish River as it approaches Puget Sound. Major tributary streams include Big Soos, Jenkins, Covington, Crisp, and Newaukum creeks. Flows in the Green River are largely controlled by Howard Hanson Dam located in the upper watershed.

Existing water rights

Water rights have been issued in the Duwamish-Green Watershed for over 100 years, and as a result most water in the watershed is already legally spoken for or “appropriated.”

Instream flow rule

Instream flow rules are an element of water and river management that help maintain healthy ecosystems that support fish, communities, and economies. WRIA 9 has an instream flow rule (WAC 173-509), adopted to preserve the uses and values of individual rivers and streams.

Streamflow restoration

The WRIA 9 Watershed Restoration and Enhancement Committee submitted a locally approved watershed plan to Ecology on February 24, 2021. We are reviewing the plan and will make a decision soon on whether to adopt it. The watershed plan forecasts the number of new permit-exempt wells between 2018 and 2038, and their potential impacts on streamflows. The plan recommends projects and actions that, if completed, will offset the impact of additional wells and provide a net ecological benefit to the watershed.

Go deeper

Find more information about water availability in WRIA 9 including acquiring water, finding existing water rights and learning more about instream flow rules in our Water Availability Focus Sheet linked below.