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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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CSO Advisory: Snowmelt and rain lead to combined sewer overflow (CSO) to the Willamette River

(January 18, 2017) - Snowmelt combined with heavy rain caused Portland’s combined sewer system to overflow to the Willamette River this afternoon at several locations.

Because of increased bacteria in the water, the public should avoid contact with the Willamette River for 48 hours after the CSO event ends. The event began at 1:10 p.m. and is still continuing. The volume is not yet known.

The overflows begin at the outfall near the Ross Island Bridge and extend downstream. Several outfalls are affected.

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 storms, the big pipes store large quantities of stormwater and sewage while pumping it to the Columbia Boulevard Wastewater Treatment Plant. During very heavy storms, some combined sewage can overflow. A combined sewer overflow is about 80% stormwater and 20% sanitary sewage.

This is the second CSO this 2016-2017 winter season. The previous overflow occurred on Thanksgiving Day. 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


Media Contact Diane Dulken, 503-823-5328,