Skip to Main Content View Text-Only

Environmental Services

working for clean rivers

Phone: 503-823-7740

Fax: 503-823-6995

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

More Contact Info

Combined Sewer Overflows (CSOs)

Combined Sewer Overflows (CSOs)

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 -  Acombined 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. The 14-minute CSO began at 7;32 pm and discharged about 23,000 gallons into the Willamette River.  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. Overflows occurred at 4:15 a.m., 2:33 p.m., and 3:01 p.m. to the Willamette River from the SE Alder Street Pump Station at SE Alder Street and Water Avenue. 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.  

Nov 24, 2016 - The CSO advisory issued on Thanksgiving Day, November 24, is no longer in effect (click here for details).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.