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

Press Release

This advisory will end at 6:15 p.m. December 29, or 48 hours after the end of the overflow.

With rains expected to continue over the next few days, you can follow the Big Pipe system in real time by visiting the Big Pipe Tracker

Heavy rains led the Big Pipe system to reach capacity around 10:15 p.m. this evening, leading to a combination of stormwater and sewage overflowing to the Willamette River from multiple outfalls. The combined sewer overflow (CSO) affects the stretch of the river downstream of Willamette Park; the overflow is ongoing at this time. 

Circle showing Big Pipe system at zero percent full. Chart below shows levels in the system over 72 hours, starting at 40% full, dropping to zero and then climbing to 100 percent starting around 11 pm Dec. 26 and continuing through about midnight Dec. 27 before dropping to zero by 3 pm Dec. 28
A 72-hour view of the Big Pipe system during this storm

During an overflow and for 48 hours afterward, the public is advised to avoid contact with the river because of increased bacteria in the water. The river’s water quality is safe for recreation during all other times. 

A CSO is about 80 percent stormwater and 20 percent sewage. CSOs are rare and can occur during periods of heavy rain or snowfall. The public can follow the duration of this overflow by viewing the Big Pipe Tracker online.

Since completing the Big Pipe project in 2011, the 20-year $1.4 billion program to reduce overflows, the number of CSOs have dropped by 94 percent to the Willamette River and 99 percent to the Columbia Slough. Before the project, CSOs occurred to the Willamette River an average of 50 times a year, with some instances lasting days. Today, overflows occur an average of four times per winter season and once every three summers. This is the fourth CSO of the year. 

The Big Pipe system refers to a series of improvements, from disconnecting downspouts on homes to allow rainwater to be absorbed naturally in the ground to the construction of big pipes on both sides of the river and along the slough to store and convey large quantities of flows to the City’s main wastewater treatment plant in North Portland. Visit About CSOs to find out more.

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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