CSO Advisory: Heavy rain leads to combined sewer overflow to the Willamette River

This advisory expired at 2 am on Sunday, December 21, 2025.

About the CSO

• The CSO affects the stretch of the Willamette River downstream of the Ross Island Bridge.
• The overflow started around 4:20 pm on December 18 and is ongoing at this time.
• This advisory may be adjusted as heavy rains continue.

Public advisory

• Avoid contact with the river in the affected area during the overflow and for 48 hours after it stops due to increased bacteria levels.
• Recreational activities such as swimming, paddleboarding, kayaking, and other activities with contact with the river are not advised during this time.

About CSOs

• CSOs can occur during periods of heavy rain or snowfall.
• Combined sewage from a CSO is approximately 80% stormwater and 20% sewage.
• Portland's Big Pipe Project has reduced CSOs by 94% to the Willamette River and 99% to the Columbia Slough.
• Overflows used to occur 50 times a year, sometimes lasting for days, but now occur 4 times a year on average.
• Before the Big Pipes, as little as 0.10 inch of rain in 24 hours could trigger a CSO.
• At the time this advisory was posted, Portland had received nearly 1.5 inches of rain since 3 a.m. December 18.

Stay updated and learn more

• Track the overflow or how the Big Pipes are keeping combined sewage out of the Willamette River in real time by following the Big Pipe Tracker.
• Learn more about CSOs and the Big Pipe Project.