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PORTLAND, Ore. (Portland Tribune) — A wintry mix of heavy rain and snow caused the Big Pipe combined sewer system in Portland to reach capacity and overflow into the Willamette River about 2 a.m. Sunday, Dec. 19.

It was the fifth overflow into the river in 2021 and lasted about four hours. The overflow happened a little more than a month after the Bureau of Environmental Services, which operates the sewer system, launched on online tracker that allows the public to monitor the remaining capacity of the system. It is updated every 15 minutes and can be found at www.portland.gov/bes/big-pipe-tracker.

Combined sewer overflows (CSO) are about 80% storm water and 20% sewage. They are rare and can occur during periods of heavy rain or snowfall.

During any overflow and for 48 hours afterward, 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.

According to the environmental services bureau, 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% to the Willamette River and 99% to the Columbia Slough.

Before the project, CSOs occurred in 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.

More about the Big Pipe and CSOs can be found at www.portland.gov/bes/about-csos.