CSO Advisory Notifications: If a rare CSO does occur, be one of the first to know by subscribing to our Combined Sewer Overflow Advisories email notification list.
What are CSOs?
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 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.
CSO and Capacity, Management, Operation, and Maintenance
Each year, Environment Services submits a report to the Oregon Department of Environmental Quality summarizing the performance of the City’s CSO and Capacity, Management, Operation, and Maintenance (CMOM) programs. Both programs focus on reducing the likelihood of sewer overflows or releases by maintaining and improving the overall reliability of the sanitary and combined sewer collection systems. Annual reports are required as part of the City’s NPDES permit. Reports summarize performance for the fiscal year (July 1 through June 30) and include information on:
- Major storm events that caused CSOs
- Wet weather treatment performance at the plant
- Water quality sampling for the Willamette and Columbia Rivers
- Tracking and reporting of sewer releases
- Collection system storage capacity
- System risk reduction and reinvestment
Annual Reports
Past CSOs
| Year | Date of CSO | Part of the Big Pipe System (Yes/No) | Impacted Waterway |
|---|---|---|---|
| 2025 | February 23 | Yes | Willamette River |
| 2024 | January 27 | Yes | Willamette River |
| 2023 | March 13 | Yes | Willamette River |
| May 8 | No | Willamette River | |
| October 10 | No | Willamette River and Columbia Slough | |
| December 6 | Yes | Willamette River | |
| 2022 | January 3 | Yes | Willamette River |
| April 30 | No | Willamette River | |
| June 11 | No | Willamette River | |
| November 4 | Yes | Willamette River | |
| December 26 | Yes | Willamette River | |
| 2021 | January 12 | Yes | Willamette River |
| September 19 | No | Willamette River | |
| September 27 | No | Willamette River | |
| November 12 | Yes | Willamette River | |
| December 19 | Yes | Willamette River | |
| 2020 | December 20 | Yes | Willamette River |
| 2019 | August 10 | No | Willamette River |
| 2018 | April 8 | Yes | Willamette River |
| October 27 | No | Willamette River | |
| 2017 | January 18 | Yes | Willamette River |
| February 5 | Yes | Willamette River | |
| February 8 | No | Willamette River | |
| February 16 | Yes | Willamette River | |
| May 13 | No | Willamette River | |
| September 20 | Yes | Willamette River | |
| October 19 | Yes | Willamette River | |
| 2016 | May 19 | Yes | Willamette River |
| October 14 | Yes | Willamette River | |
| November 24 | Yes | Willamette River | |
| 2015 | January 17 | Yes | Willamette River |
| March 15 | Yes | Willamette River | |
| October 31 | Yes | Willamette River | |
| November 17 | Yes | Willamette River | |
| December 7 | Yes | Willamette River and Columbia Slough | |
| December 17 | Yes | Willamette River | |
| 2014 | March 28 | Yes | Willamette River |
| June 16 | Yes | Willamette River | |
| October 22 | Yes | Willamette River | |
| December 4 | Yes | Willamette River | |
| 2013 | May 23 | Yes | Willamette River |
| September 27 | Yes | Willamette River | |
| 2012 | January 18 | Yes | Willamette River |
| May 26 | Yes | Willamette River | |
| November 19 | Yes | Willamette River and Columbia Slough | |
| November 24 | Yes | Willamette River |