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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 Cancelled: Overflow did not occur to the Willamette River today

UPDATE 4 P.M. SEPTEMBER 14, 2018 - This advisory is cancelled. Environmental Services' inspection determined that there was no combined sewer overflow to the Willamette River last night. The original advisory, issued early this morning, was based on an automated monitoring system but field inspectors today determined that  all stormwater and sewage was contained. There is no overflow. Enjoy your river.

Original headline: CSO Alert: 5-minute combined sewer overflow (CSO) occurs to Willamette River near Cathedral Park; avoid contact downstream for 48 hours

(September 14, 2018) – An isolated rain burst last night near the east end of the St. Johns Bridge led to a short combined sewer overflow (CSO) from an outfall near Cathedral Park.

The rain burst dumped four-tenths of an inch within 15 minutes.  The overflow lasted about five minutes and led to an estimated 2,500 gallons of combined stormwater and sewage to overflow to the river. 

The majority of the discharge is stormwater, mixed with about 20 percent sewage. 

CSOs are rare. But when they occur the public is advised to avoid contact with the river water for 48 hours due to increased bacteria. In this case, the area affected is at Cathedral Park and downstream.

Since completing the Big Pipe project in 2011, a 20-year $1.4 billion program to reduce overflows, the number of CSOs have been reduced 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.

For more information about CSO events, what they are and why they occur, visit

Media contact: Diane Dulken (503)457-7636