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The City of Portland, Oregon

Environmental Services

working for clean rivers

Phone: 503-823-7740

Fax: 503-823-6995

1120 SW 5th Ave, Suite 613, Portland, OR 97204

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CSO Advisory: Heavy rains Saturday led to combined sewer overflow (CSO) at St. Johns Bridge

(October 29, 2018) - Heavy rains led to a combined sewage overflow late Saturday night from a single outfall near the St. Johns Bridge to the Willamette River.

While Portland experienced heavy rains throughout the city this weekend, rain fall was especially intense in St. Johns. The overflow was brief; the volume is unknown at this of outfalls

Because of increased bacteria in the water, the public should avoid contact with the Willamette River at Cathedral Park and downstream of the St. Johns Bridge through midnight tonight.

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, a 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. 

The Big Pipe project constructed 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.

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 second overflow in 2018.

Find out more information about CSO events, what they are and why they occur at

The Bureau of Environmental Services provides city 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. Visit and follow us on Twitter for news @besportland.

Media contact: Diane Dulken 503-457-7636 or