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Environmental Services

working for clean rivers

Phone: 503-823-7740

Fax: 503-823-6995

1120 SW 5th Avenue, Room 1000, Portland, OR 97204

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Combined sewer overflow (CSO) to the Willamette River

Update: October 17, 3:54 p.m. - This CSO advisory is no longer in effect.

The Alder Pump Station couldn't handle a surge of combined sewage and stormwater during a 20-minute period of intense rainfall on Friday, October 14, resulting in about 30,000 gallons discharging to the river from an outfall pipe. This overflow was just the second of 2016.

CSO Advisory

October 14, 2016

Combined sewer overflow (CSO) to the Willamette River

Heavy rain this evening caused Portland’s combined sewer system to overflow from the Alder Pump Station at SE Alder Street and SE Water Avenue to the Willamette River. High combined flows of sewage and stormwater due to heavy rainfall in a short period of time exceeded the capacity of the pump station to send the flows to the Columbia Boulevard Wastewater Treatment Plant.

Because of increased bacteria in the water, the public should avoid contact with the Willamette River from the Morrison Bridge to the Columbia River confluence for 48 hours after the CSO event ends. The event began at 6:58 p.m. this evening and is still occurring. The volume is not known.

Portland’s combined sewer system carries sewage and stormwater runoff in the same pipes. In December 2011, Portland completed a 20-year program of sewer improvements, including constructing big pipes on both sides of the Willamette River and along the Columbia Slough. The improvements eliminate 99% of CSOs from the slough and 94% from the river.

During extremely heavy rain, the big pipes store large quantities of stormwater and sewage while pumping it to the Columbia Boulevard Wastewater Treatment Plant. During very heavy rain storms, some combined sewage can overflow. A combined sewer overflow is about 80% stormwater and 20% sanitary sewage.

Before the city completed the CSO control program, combined sewers overflowed an average of 50 times a year. Today, the combined system overflows to the Willamette River an average of four times per winter and once every three summers.

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

Contact Cheryl Kuck, 503-823-7898,

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.