Localized Heavy Rains Lead to a Combined Sewer Overflow to the Willamette River

News Article

Heavy rains led to a combined sewer overflow (CSO) to the Willamette River from an outfall located just north of Willamette Park. The overflow was short in duration, beginning at 7:34 p.m. and ending at 7:36 p.m. on Saturday, June 11. The volume of combined sewage that reached the river was about 344 gallons. This outfall is not part of the Big Pipe system. There were no CSOs from the Big Pipe System despite the unseasonally late atmospheric river that impacted the area late last week and this weekend.

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.

A 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.

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.

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Environmental Services protects public health and our environment as Portland’s sewer and stormwater utility. We collect and recover resources from wastewater. We prevent pollution and flooding from stormwater. We protect and restore Portland’s rivers, streams, and watersheds for all Portlanders — now and for future generations. Follow our news on Twitter @BESPortland and portland.gov/BES/news