danger
COVID-19 Risk Level for Multnomah County: Lower Risk
information
Regional chlorine shortage: Portland water remains safe to drink

A regional chlorine shortage has disrupted supplies. The Portland Water Bureau is monitoring the situation and evaluating our supplies and procedures. Get updates.

About Combined Sewer Overflows or CSOs

Information
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.

No CSO advisory is currently in effect.


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.

Overflows: 

  • 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 chemicals and bacteria that could harm people and animals in the water. 

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