CSO Advisory: Atmospheric river dashes hope that 2020 would set a record for zero overflows to the Willamette River

News Article

Update: Preliminary estimates show that today’s overflow lasted three hours, ending at 2:15 p.m. The advisory to avoid contact with the river water remains in effect for 48 hours.

Original news release:

With 11 days left to the year, this weekend’s storm led to a combined sewer overflow (CSO) on Sunday morning to the Willamette River. 

The overflow is the first of 2020, and the first to affect the entire stretch of Portland’s Willamette River since April 2018.This overflow is occurring from eight outfalls located between the Ross Island Bridge and Swan Island. It began at about 11 a.m. and is still continuing. 

During an overflow, and for 48 hours afterwards, the public is advised to avoid contact with the river because of increased bacteria in the water. During all other times, the river’s water quality is safe for recreation. A combined sewer overflow is about 80 percent stormwater and 20 percent sewage.

Overflows that used to be common are now rare, ever since Environmental Services completed the Big Pipe Project in December 2011 with a target of eliminating 94 percent of such incidents. 2020 had been on track to exceed that target and be the first calendar year in modern Portland history with zero incidents.  

“Overflows will continue to be rare well after 2020 is over,” saidEnvironmental Services engineer Arnel Mandilag, “Portlanders have invested in the health of the Willamette River. That investment is making overflows the exception rather than the rule, creating a healthier river for wildlife and people.”

The last overflow was August 2019, a 15-minute incident from a single outfall near Sauvie Island from a hyper-local storm. The last overflow thatlike this weekend’s, affected Portland’s entire stretch of the river was in April 2018. 

Before the Big Pipe Project, overflows would occur with rainfall as little as one-tenth of an inch in a day, which regularly happens in Portland. Overnight Saturday into Sunday, this storm’s rainfall exceeded one inch in 12 hours and is continuing.

The Big Pipe Project greatly expanded the system’s capacity to keep the river sewage-free. It is named after its main features - two giant pipes on either side of the river that carry a combination of sewage and stormwater to the City’s main wastewater treatment plant. It took a generation to build - 20 years - and included many related projects, from nature-based solutions such as rain gardens and disconnected downspouts that let rain soak into soil, to complex new pipes and pumping systems. In addition to the Willamette River, a similar pipe along the Columbia Slough has reduced overflows there by 99 percent.

Follow updates on Twitter @BESPortland. Previous news release.

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The City of Portland Bureau of Environmental Services - your sewer and stormwater utility - provides Portland 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. 


Diane Dulken

Public Information Officer