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Environmental Services

working for clean rivers

Phone: 503-823-7740

Fax: 503-823-6995

MAILING ADDRESS: 1120 SW 5th Ave, Room 1000, Portland, OR 97204

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Combined Sewer Overflows (CSOs)

Combined Sewer Overflows (CSOs)

The following are advisories for 2017. All advisories also are posted on BES's news page

Oct. 22, 2017 - Heavy rains led to a CSO from multiple locations. A CSO advisory is in effect for 48 hours. 

Oct. 19, 2017 -  A 14-minute-long combined sewer overflow (CSO) occurred this evening from a single location - the SE Alder Pump Station that is slated to be taken offline later this month to improve and expand its capacity.  A CSO advisory is in effect for 48 hours downstream from the Morrison Bridge. 

Sept. 20, 2017 - Heavy rains caused Portland’s combined sewer system to overflow to the Willamette River from a single location - the SE Alder Street Pump Station.. A CSO advisory is in effect for 48 hours downstream from the Morrison Bridge.

May 13, 2017 - A CSO advisory for the Willamette River is in effect for 48 hours, following a ten minute overflow from one outfall north of Willamette Park after heavy thunderstorms and a suspected debris blockage at the outfall. No other outfalls are affected.

Feb. 16, 2017 - A CSO advisory for the Willamette River went into effect for 48 hours, following an overflow that began around noon. 

Feb. 5, 2017 - A CSO advisory  for the Willamette River went into effect for 48 hours following a storm that dropped about 2.5 inches on Portland

Jan 18, 2017 - A CSO advisory for the Willamette River was issued for 48 hours due to heavy snowmelt and rain.   

Background and Useful Facts

The Big Pipe Project -   Portland’s $1.4 billion Big Pipe Project, completed in 2011 after 20 years, has dramatically reduced combined sewer overflows to the Willamette River and Columbia Slough.

* The improvements eliminate 94% of combined sewer overflows to the Willamette River and 99% to the Columbia Slough.

* Eliminating 100% of overflows would have doubled the $1.4 billion project cost without significantly improving river health, according to city projections that were reviewed by the state Department of Environmental Quality and federal Environmental Protection Agency.

* Before the project, combined sewer overflows occurred an average of 50 times a year. Today, overflows to the Willamette River occur - on average -  four times per rainy season (November through April) and once every three years during the dry season.

*  There have been zero overflows since 2011 during the peak recreation months of July and August (this information is current through 2017).

* A combined sewer overflow is about 80% stormwater and 20% untreated sewage.

* How the system works: The city's combined sewer system carries sewage and stormwater runoff in the same pipes. The Big Pipe Project - also known as the Combined Sewer Overflow Control Program - got its name from the three big pipes that expanded the city's ability to intercept combined sewage and stormwater and direct the flow to the treatment plant.The pipes are on both sides of the Willamette River and along the Columbia Slough.

 Find out more at