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

Press Release

December 8, 2023 Update: This advisory has expired.

Heavy rains led the Big Pipe system to reach capacity around 7 a.m., 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 Sellwood Park. The overflow is ongoing at this time.

During an overflow and for 48 hours after an overflow stops, 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, incidents occurred to the Willamette River an average of 50 times a year, with some 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.  As of yesterday at noon, the Big Pipe system had captured and prevented about 300 million gallons of diluted sewage from reaching the river since this week’s series of storms began. There is no immediate estimate for today’s overflow. 

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.

About Environmental Services

Environmental Services - the City of Portland’s sewer and stormwater utility - protects public health and environment by collecting and recovering resources from the city’s wastewater, managing stormwater, and restoring and protecting Portland’s rivers, streams, and watersheds.


Diane Dulken

Public Information Officer