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The City of Portland, Oregon

Environmental Services

working for clean rivers

Phone: 503-823-7740

Fax: 503-823-6995

1120 SW 5th Ave, Suite 613, Portland, OR 97204

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Combined sewer overflow (CSO) to the Willamette River

Updated: 8:45 a.m. on November 29, 2016

This CSO Advisory is no longer in effect. The combined sewer overflow began during the afternoon of November 24 (12:30 p.m. on Thanksgiving Day) and lasted for about 17 hours (7:30 a.m. on Black Friday) with all of the 8 Willamette River tunnel relief points involved in the overflow. This resulted in about 159 million gallons of combined sewage and stormwater discharging to the river.

CSO Advisory

November 24, 2016

Combined sewer overflow (CSO) to the Willamette River

Heavy rain caused Portland’s combined sewer system to overflow to the Willamette River on Thursday afternoon, Thanksgiving Day, at three locations. The overflows occurred at the Alder Pump Station on the east bank of the Willamette River approximately 100 yards south of the Morrison Bridge, the Ankeny Pump Station on the west bank of the Willamette River approximately 100 yards south of the Steel Bridge, and an outfall pipe on the east bank of the Willamette River approximately 300 yards north of the Fremont Bridge.

High combined flows of sewage and stormwater due to heavy rainfall exceeded the capacity of the pump stations to send the flows to the Columbia Boulevard Wastewater Treatment Plant in North Portland.

According to the National Weather Service, storm systems will continue to bring periods of heavy rain to the Portland area through Sunday. Because of increased bacteria in the water, the public should avoid contact with the Willamette River for 48 hours after the CSO events end. All three locations were still overflowing at the time of this advisory.

Go to for updates about the CSO events, including the time the events end.

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 heavy rain, the big pipes store large quantities of stormwater and sewage while pumping it to the Columbia Boulevard Wastewater Treatment Plant. During extremely 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.

For more information about CSO events, what they are and why they occur, visit

Contact Cheryl Kuck, 503-823-7898,

The Bureau of Environmental Services provides city residents with programs to protect water quality and public health, including wastewater collection and treatment, sewer construction and maintenance, stormwater management, and stream and watershed restoration.