CSO Advisory: Combined sewer overflow occurred late afternoon to Willamette River from a single outfall at Willamette Park

Press Release

Update: The advisory expired at 5 p.m. May 10, 48 hours after the end of the 30-minute overflow.

A thunderstorm rolling through Portland this afternoon led to a combined sewer overflow from a single outfall near Willamette Park at around 4:20 p.m. It lasted for less than half an hour.

The Big Pipe system was at 12 percent full at the time of this incident, and continues to prevent overflows from other locations.

While this is a small overflow from a single location, the public is still advised to avoid contact with the river downstream of the park for 48 hours because of increased bacteria in the water. 

With fishing season underway, Environmental Services adds that people fishing either wait for the end of the advisory period or wash their hands thoroughly if exposed to river water. 

A combined sewer overflow (CSO) is about 80 percent stormwater and 20 percent sewage. CSOs are rare and can occur during periods of heavy rain or snowfall when pipes that carry a mix of sewage and stormwater get overwhelmed. 

The Big Pipe Project completed in 2011 greatly expanded the City of Portland’s ability to prevent overflows, reducing incidents by 94% on the Willamette River and 99% to the Columbia Slough.

Overflows now occur about four times during the rainy season and once every three years during the summer. Overflows from a single location are rarer still but do occur when isolated storms severely impact a local area, as occurred this afternoon. 

Environmental Services - the City of Portland’s sewer and stormwater utility - protects public health and environment by collecting and recovering resources from the city’s wastewater, managing stormwater, and restoring and protecting Portland’s rivers, streams, and watersheds. www.portland.gov/bes.


Diane Dulken

Public Information Officer