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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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Portland’s combined sewers overflow to the Willamette River and Columbia Slough

CSO Advisory Update

December 7, 2015

Heavy, sustained rain caused Portland’s combined sewer system to overflow to the Willamette River and Columbia Slough today. The combined sewer overflow (CSO) event to the Willamette River began at about 7:50 a.m. A CSO outfall on the slough began overflowing at about 10:30 a.m. today.

Because of increased bacteria in the water, the public should avoid contact with the Willamette River from the Sellwood Bridge to the Columbia River confluence, and with water in the Columbia Slough from about N Denver Avenue to the slough’s Willamette River confluence for 48 hours after the CSO event ends. Go to for details about the CSO event, including the time the event ends.

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 a major storm event, 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. Today’s overflow to the Columbia Slough was the first caused by heavy rain since the city completed the Columbia Slough Big Pipe in 2000.

For more information contact Linc Mann, 503-823-5328.

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.