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Storm damage recovery

CSO Advisory: Strong atmospheric river leads to combined sewer overflow to the Willamette River

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November 7 Update: Last week's overflow was 5.25 hours, beginning around 9:15 pm on Friday, November 4, and ending around 2:30 am Saturday, November 5. The total volume of the overflow was an estimated 37.2 million gallons. Follow the Big Pipe Tracker to see current conditions.


November 5 Update: The CSO started around 9:15 pm and ended at 2:30 am on November 5. The public is advised to avoid contact with the river for 48 hours after the CSO has ended because of increased bacteria in the water.


Heavy rains associated with the strong atmospheric river led the Big Pipe system to reach capacity around 9:15 pm and overflow combined stormwater and sewage to the Willamette River. The combined sewer overflow (CSO) is ongoing at this time.

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.

The public can see Big Pipe levels by checking the Big Pipe Tracker online.

A 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 first CSO of this water year, which began on October 1.

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.

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.