Combined sewer overflows (CSOs) are relief points in older sewer systems that carry sewage and stormwater in the same pipe. When heavy rains fill the pipes, CSOs release sewage and stormwater into rivers, lakes, or Puget Sound.

In King County, CSOs are only located within the City of Seattle. King County and the City of Seattle are working together to control CSOs and keep sewage and stormwater out of local waterways.

Learn how often a specific CSO outfall overflows in Appendix A of the most recent Annual CSO Report.