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

Press Release
A combined sewer overflow to the Willamette River began about 9:45 p.m. January 12 and ended about six hours later. Avoid contact with the river through Friday, January 15 due to increased bacteria levels.

Tonight’s heavy rains caused Portland’s combined sewer system to overflow to the Willamette River at several outfalls downstream of Willamette Park. The overflow began around 9:45 p.m. and ended about 3:45 a.m. 

Chart of Big Pipe levels showing a mix of sewage and stormwater filling the pipes and reaching 100% (overflow level) for six hours ending around 3:45 a.m.
Big Pipe levels during the Jan.12-13, 2021 storm. When levels reach 100% an overflow occurs.

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 (CSO) is about 80 percent stormwater and 20 percent sewage.

This is the first CSO of 2021.  There was a single overflow in 2020. Overall, 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. Find out more about CSOs.

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. BES @besportland


Diane Dulken

Public Information Officer