Skip to Main Content View Text-Only

Environmental Services

working for clean rivers

Phone: 503-823-7740

Fax: 503-823-6995

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

More Contact Info

Combined Sewer Overflows (CSOs)

Controlling Combined Sewer OverflowsCSO diagram

The City of Portland completed its 20-year CSO Control Program in December 2011. The program reduced CSOs to the Columbia Slough by more than 99% and to the Willamette River by 94%. Instead of an average of 50 Willamette River CSO events each year, there are now an average of four CSO events each winter and one event every third summer during only very heavy rain storms. The city met all of its required CSO program milestones on time.

How We Got Here

Older Portland neighborhoods have a sewer system that mixes untreated sewage and stormwater runoff in a single pipe. During very heavy rain storms, runoff from buildings, streets, and other hard surfaces can fill these combined sewers to capacity and cause them to overflow.

Working for Clean Rivers

Controlling CSOs is an important part of Portland’s efforts to improve Willamette River water quality. CSO solutions included projects to remove divert runoff from sewers and building new facilities to carry sewage and stormwater to the Columbia Boulevard Wastewater Treatment Plant.

Cornerstone Projects

Portland started the CSO control program in 1991 with a set of Cornerstone Projects that remove stormwater from combined sewers. Projects included installing street sumps and sedimentation manholes, building separate sewers for stormwater in some neighborhoods, encouraging homeowners in targeted neighborhoods to disconnect downspouts from the sewer system, and removing underground streams from the combined sewers.

Big Pipe Projects

  • Environmental Services completed Columbia Slough CSO projects in 2000. They included the Columbia Slough Big Pipe, a 3.5-mile long, six and 12-foot diameter pipeline to collect combined sewage and transport it to the Columbia Boulevard Wastewater Treatment Plant. Environmental Services expanded plant capacity to accommodate the extra flow. The slough projects controlled 13 CSO outfalls and reduced CSOs to the slough by 99%.
  • Environmental Services completed the West Side Big Pipe and the Swan Island CSO Pump Station in 2006 to control 16 CSO outfalls on the west side of the Willamette River. The 3.5-mile, 14-foot diameter tunnel carries combined sewage to Swan Island and the CSO pump station pumps it to the Columbia Boulevard Wastewater Treatment Plant.
  • Also in 2006, Environmental Services began construction of the East Side Big Pipe, a six-mile long, 22-foot diameter tunnel to collect sewage from the east side of the Willamette. The city completed tunneling in October 2010 and completed connecting combined sewers to the east side tunnel in September 2011 to control the 19 remaining Willamette River CSO outfalls.

CSO Milestones

1993 - Work begins on projects to divert stormwater runoff from the combined sewer system. The sump installation, sewer separation, stream diversion and residential downspout disconnection projects are known collectively as the Cornerstone Projects.

2000 - Environmental Services completes construction of the Columbia Slough Big Pipe and other slough projects to reduce CSOs to the slough by more than 99%.

2006 - The 14-foot diameter West Side Big Pipe and Swan Island Pump Station are completed, and work starts on East Side CSO projects.

2007 - The Downspout Disconnection Program disconnects downspout number 50,000, and disconnections are removing more than one billion gallons of stormwater annually from the combined sewer system.

2010 - Tunneling is completed on the East Side Big Pipe Project.

2011 - Work is complete on the East Side Big Pipe, the Balch Consolidation Conduit and the Sellwood CSO Projects.

Green Solutions

Environmental Services promotes other innovative projects to manage stormwater onsite instead of piping it into sewers or streams. Projects include ecoroofs, green streets, rain gardens, swales and stormwater planters.

Community Benefit Opportunity (CBO) Program

In November 2007, the Portland City Council authorized spending $1.77 million in Community Benefit Opportunity (CBO) Program funds on community projects in areas near East Side Big Pipe construction. The city created the CBO Program to add amenities to neighborhoods affected by CSO construction.

East Side CSO construction affected 11 neighborhoods between SE 17th and McLoughlin Boulevard and Swan Island. Community groups and citizens in those areas nominated 38 projects. A citizens' advisory committee reviewed the proposals, and Environmental Services recommended 21 projects for funding.

Projects include bank restoration along the Willamette River, street tree planting, community gardens, and sustainable stormwater management facilities.