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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 Control

Portland's Combined Sewer System

Portland CSO control system mapPortland's first sewers, constructed in the late 19th century, combined sewage and stormwater in the same pipes and carried the wastewater directly to the Willamette River and Columbia Slough without treatment. In 1952, new interceptor pipes began collecting sewage from the combined sewers for conveyance to Portland's first sewage treatment plant.

The new system improved water quality. But because stormwater runoff during wet weather filled the combined sewers to capacity, some of the sewage and stormwater mixture still overflowed to the river and slough. New streets, rooftops and other hard surfaces increased as Portland grew, generating more stormwater runoff and increasing CSO volume.

Estimated annual CSO volume reached 10 billion gallons by the 1970s. A series of sewer improvement projects reduced volume to about six billion gallons a year by 1990.

The City of Portland and the Oregon Department of Environmental Quality (DEQ) agreed on a CSO control plan in 1991, and an amended plan in 1994. That 1994 "Amended Stipulation and Final Order (ASFO)" required Portland to control CSOs to the Columbia Slough by 2000 and to the Willamette River by 2011.

The city finished CSO construction and activated the Willamette River CSO tunnel system before its December 1, 2011 deadline.

Before the CSO program began, Portland’s combined sewers overflowed an average of 50 times a year, spilling about six billion gallons of combined sewage into the Columbia Slough and Willamette River every year. The CSO program reduced overflows to the slough by 99% and to the river by 94%. Today, combined sewers overflow to the river no more than four times per winter and once every three summers.

As little as one-tenth of an inch of rain in 24 hours once caused a CSO event. Now it takes a winter storm of at least 1 inch in 24 hours and a summer storm of 1.4 inches in 24 hours to cause a CSO.


Controlling Combined Sewer Overflows

How Portland controls CSOs

Watch "Portland CSO Program 1991-2011"

A video overview of Portland's 20-year combined sewer overflow control program

Cornerstone Projects

Downspout disconnection, sump installation, stream diversion and sewer separation are the cornerstones of the CSO control program

Columbia Slough Projects

Projects controlled CSOs to the slough by 99%

West Side Big Pipe

Environmental Services completed the West Side Big Pipe projects in 2006

Watch "A River Renewed"

2004 CSO Program video narrated by Walter Cronkite

East Side Big Pipe Project

Environmental Services completed East Side Big Pipe construction in 2011

River Alert Program

Check to see if a combined sewer overflow (CSO) has occurred in the Willamette River

Sustainable Stormwater Management

Stormwater management that mimics natural systems