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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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What did the CSO program accomplish?

Controlling CSOs makes the Willamette River and Columbia Slough much healthier for people, fish and wildlife. People can safely enjoy recreation on the river and slough on all dry days, and all but the stormiest wet days.

Before CSO control Today, after CSO control
Average of 50 CSO events per year Average of 4 to 5 CSO events per year
Each CSO event discharged an average of 125 million gallons of combined sewage; each event lasted almost two days A CSO event ranges from a few hundred thousand gallons to over 100 million gallons during very heavy rainstorms; events last from a few minutes to a few hours
There were 54 active outfall pipes along the Willamette River and Columbia Slough There are 25 active CSO outfalls, 16 on the river and 9 on the slough
One-tenth of an inch of rain in 24 hours could cause a CSO One inch of rain in 12 hours may cause a CSO to the Willamette

stormy day on the WillametteWhat kind of weather can cause a CSO event?

Environmental Services used historical rain data when designing the CSO system to determine the size of storms that will cause an overflow.

  • Today, there is a 10% chance of a CSO to the Columbia Slough in summer and a 20% chance of a CSO to the slough in winter.

  • For the Willamette River, there is a 67% chance of a CSO in the winter. The system is sized to overflow during large storms that typically happen four times every winter. There is a 33% chance of an intense storm causing an overflow in the summer, or an average of one every three summers.

During a CSO event, the system operates the way Environmental Services designed it. The big pipes collect and store combined sewage. Most of it goes to the wastewater treatment plant. Some stormwater mixed with diluted, untreated sewage can overflow to the river during very heavy rainstorms.