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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: Heavy rains lead to combined sewer overflow (CSO) to the Willamette River

Updated: Environmental Services has determined that a total of about 90 million gallons of combined sewage (80 percent stormwater, 20 percent sewage) was released into the Willamette River beginning at noon and ending at 7:30 p.m. on February 16.

(February 16, 2017) - Heavy rains caused Portland’s combined sewer system to overflow to the Willamette River beginning at noon 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. Find out more about CSOs, what they are and why they occur at:

This is the fifth CSO this 2016-2017 winter season. Previous overflows this season occurred on February 9 for 13 minutes due to a debris blockage in an outfall that quickly cleared itself; on Super Bowl Sunday, February 5, due to unusually heavy rains that dropped about 2.5 inches within 24 hours; on January 18  due to a combination of snowmelt and heavy rain; and Thanksgiving Day due to heavy rain.

Since completing a CSO control program in December 2011, the city has eliminated 94 percent of overflows to the Willamette River and 99 percent from the Columbia Slough.  

Portland’s combined sewer system carries sewage and stormwater runoff in the same pipes. In December 2011, Portland completed the 20-year program of sewer improvements, including constructing big pipes on both sides of the Willamette River and along the Columbia Slough. 

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 percent stormwater and 20 percent sanitary sewage. 

Before completing the 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.

The Bureau of Environmental Services works with Portland residents and businesses to protect water quality, public health, and the environment through wastewater collection and treatment, sewer construction and maintenance, stormwater management, and stream and watershed restoration