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Environmental Services

working for clean rivers

Phone: 503-823-7740

Fax: 503-823-6995

1120 SW 5th Avenue, Room 1000, Portland, OR 97204

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Combined Sewer Overflows (CSOs)

Combined Sewer Overflows (CSOs)

Jan 18, 2017 - A combined sewer overflow (CSO) advisory went into effect this afternoon for the Willamette River. Updates will be  posted. 

The CSO advisory issued on Thanksgiving Day, November 24, is no longer in effect (click here for details).

Portland’s combined sewer system carries sewage and stormwater runoff in the same pipes. In December 2011, Portland completed a 20-year program of sewer improvements, including constructing big pipes on both sides of the Willamette River and along the Columbia Slough. The improvements eliminate 99% of CSOs from the slough and 94% from the river.

During extremely heavy rain, the big pipes store large quantities of stormwater and sewage while pumping it to the Columbia Boulevard Wastewater Treatment Plant. During very heavy rain storms, some combined sewage can overflow. That is what the system is designed to do and that is the way it operates.

A combined sewer overflow is about 80% stormwater and 20% sanitary sewage. Before the city completed the CSO control program, combined sewers overflowed an average of 50 times a year. Today, the combined system overflows to the Willamette River an average of four times per winter and once every three summers.