Combined Sewer Overflows are extremely rare thanks to the Big Pipe Project that has eliminated 94% of overflows to the Willamette River and 99% to the Columbia Slough. When they occur, CSO advisories are posted here, as a news release on portland.gov/news and on Twitter @BESPortland.
2021 CSO Advisories
January 12, 2021 - A CSO Advisory is in effect after heavy rains lead to an overflow.
2020 CSO Advisories
Dec 20, 2020 - Atmospheric river dashes hope that 2020 would set a record for zero overflows to the Willamette River. A CSO advisory is in effect for 48 hours downstream of Ross Island Bridge.
2019 CSO Advisories
Aug 10, 2019 - Heavy rains on Saturday led to an overflow from a single outfall at NW 110th Avenue and NW Front Avenue. A CSO advisory is in effect in the vicinity and downstream through Monday night.
2018 CSO Advisories
Oct 29, 2018 - Heavy rains on Saturday night led to an overflow from a single outfall near the St. Johns Bridge. A CSO advisory is in effect downstream of the bridge through Monday night.
April 8, 2018 - Heavy rains led to combined sewer and stormwater overflows to the Willamette River. A CSO advisory is in effect for 48 hours.
Background and Useful Facts on CSOs
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 the Big Pipe completion during the peak recreation month of July and only one in August - that 2019 incident occurred for about 15 minutes from a single outfall in the Linnton area of North Portland.
* 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 https://www.portlandoregon.gov/bes/31030
2017 CSO Advisories
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