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

Update (February 8, 2107) -

This CSO Advisory ended the morning of Feb. 7, 48 hours after the overflows stopped. With Super Bowl Sunday’s storm dropping about 2.5 inches of rain in 24 hours, a combined sewer overflow began at about 1:45 pm on Sunday, Feb. 5, 2017 and lasted for about 12 hours.  Overflows occurred from most, if not all, eight Willamette River tunnel relief points, sending a mix of about 80 percent stormwater and 20 percent sewage of about 206 million gallons into the Willamette River. Since Portland’s CSO control program was completed in 2011, the number of CSOs have dropped from about 50 per year to about four during a winter season. This is the season’s third overflow.

Original message below:

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

(February 5, 2017) - Heavy rains 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 about 1:30 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.

This is the third CSO this 2016-2017 winter season. The previous overflows occurred on January 18, 2017 and 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.

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.


Find out more information about CSO events, what they are and why they occur.

 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. www.portlandoregon.gov/bes @besportland

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Media Contact Diane Dulken, 503-823-5328, diane.dulken@portlandoregon.gov