CSO Advisory: Atmospheric river leads to combined sewer overflow to Willamette River

Press Release
An advisory is in effect through 4:30 p.m. Sunday, November 14, 2021; avoid contact with river water downstream of the Ross Island Bridge due to increased bacteria.

Update: The combined sewer overflow that began around 6:30 a.m. Friday, November 12 ended around 4:30 p.m. The public is advised to avoid contact with the Willamette River downstream of the Ross Island Bridge for 48 hours after the end of the overflow, or until 4:30 pm. Sunday, November 14, to prevent exposure to elevated levels of bacteria in the water. 

The atmospheric river and accompanying heavy rains in Portland led to the Big Pipe system to reach capacity early this morning in containing combined stormwater and sewage. An overflow began to the river at about 6:30 a.m. and is 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.

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

A chart and graphic show levels of the Big PIpe at 100%
The Big Pipe Tracker shows how full the system is as the big pipes contain stormwater and sewage. An overflow occurs when the levels reach 100%

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. 

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. This is the fourth CSO of the year.

Find out more about the Big Pipe and CSOs.

Track how the Big Pipe system fills during storms.

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. 


Diane Dulken

Public Information Officer