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South Henderson Basins

Henderson Basin Map

The Henderson projects cover two areas: North Henderson and South Henderson. Basins 46, 47, 49 and 171 are defined as South Henderson. The purpose of the South Henderson area projects are to reduce the amount of untreated sewage and polluted stormwater that is overflowing into Lake Washington at combined sewer overflow outfalls. Improving water quality in the Henderson Basins will benefit the environment and recreation.

North and South Henderson combined sewer overflow (CSO) reduction projects incorporate additional storage, transmission pipeline and system upgrades to control about 3 million gallons that currently overflow into Lake Washington during storm events. The projects are planned for completion in 2018. When finished, the Henderson projects will bring six combined sewer basins into control. Improving water quality in the Henderson Basins will benefit the environment and recreation.

Because of topography and other factors, Green Stormwater Infrastructure (GSI) opportunities are limited in South Henderson Basins. Some opportunity may exist for small-scale projects, such as the RainWise Program.

Project Updates

South Henderson

Multiple alternatives to reduce CSOs in South Henderson basins are being analyzed in order to meet regulatory requirements. In order to understand the feasibility of specific alternatives, SPU is studying the conditions of the basin. As a result, you may notice SPU conducting field work in South Henderson Basins. Check back for the latest project information.

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Lower Mapes Creek Restoration Project

In July 2011, SPU found that it could reduce costs of the South Henderson Combined Sewer Overflow Basin 47B/171 Project and ensure the project would be constructed by completing the Lower Mapes Creek Restoration Project in combination with the CSO project. For more information, view Lower Mapes Creek Restoration Project.

South Lake Washington Sewage Overflow Prevention Project

(Rainier Ave. S. between 57th Ave. and 68th Ave. S)

In 2012, nearly 2 million gallons of raw sewage and stormwater overflowed into Lake Washington from an outfall located near Rainier Avenue South and South Pilgrim Street. SPU is currently considering four options to help prevent future sewage overflows in this area. SPU held a public open house on Thursday, March 6 to provide information and solicit feedback on the options currently being considered. In case you missed the meeting, below are links to the meeting materials.

Project Fact Sheet

For more information, contact Project Manager Rick Johnson at (206) 233-7861 or

Project Schedule

The State Department of Ecology has set the following schedule for South Henderson CSO Reduction Projects:

  • Complete and submit a draft Engineering Report by August 30, 2012
  • Complete and submit for approval a final Engineering Report by January 31, 2013
  • Complete and submit draft plans and specifications by October 31, 2014
  • Complete and submit for review and approval final plans and specifications by March 30, 2015
  • Begin construction by May 31, 2015

Final Reports


Contact the Henderson Basin project team.

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