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About Combined Sewer Overflows or CSOs

Combined sewer overflows to the Willamette River and Columbia Slough are rare. But when they do occur, Portlanders are advised to stay out of the water for 48 hours following a CSO due to higher levels of bacteria and other contaminants. Find out if a CSO is happening now.

Many of Portland’s older neighborhoods have a combined system. In a combined system, sewage and stormwater are carried in the same pipes to the treatment plant. Before the Big Pipe Project was completed in 2011, these neighborhood pipes used to fill up and overflow to the Willamette River or Columbia Slough when it rained. Now the Big Pipe Project has eliminated 94 percent of overflows to the Willamette and 99 percent to the slough.


  • Are rare. They occur about four times a rainy season during or after heavy rains. 

  • Usually happen in the winter from October to May. 

  • Contain about 20 percent sewage and 80 percent stormwater. 

  • Carry E. Coli bacteria that can be harmful to people, which is why people are advised to avoid contact with the water for 48 hours after an overflow.  

See related News & Notices on this page for past CSO events since 2018.