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CSO Advisory: Wintery storm leads to combined sewer overflow to Willamette River

Press Release

This CSO Advisory expired 48 hours after the overflow ended, or at about 6 a.m. December 21. Total number of hours for the overflow: 2.5 hours (original advisory estimated duration at four hours). Total volume of overflow: About 2 million gallons (for context, the wastewater treatment plant received and treated 300 million gallons of stormwater and sewage for the 48 hour period of Dec. 18 through Dec. 20). Follow the Big Pipe Tracker to see current conditions.

Original advisory: A wintery mix of heavy rain and snow overnight led the Big Pipe system to reach capacity in containing combined stormwater and sewage. An overflow to the Willamette River began about 2 a.m. and lasted around four hours.

Graph showing the big pipe filling up to 100 percent full before dropping back down to 80 percent full.

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.

The public can see Big Pipe levels at the new online tracker.

A combined sewer overflow (CSO) is about 80 percent stormwater and 20 percent sewage. 

CSOs are rare and can occur during periods of heavy rain or snowfall. This is the fifth overflow of 2021.

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 from multiple outfalls 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.

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 Columbia Boulevard Wastewater Treatment Plant.

Find out more about the Big Pipe and CSOs

Follow updates on Twitter @BESPortland.

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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.


Andy Kiemen

Communications, Environmental Services

Diane Dulken

Public Information Officer