Portland, Ore. — Around 5:15 p.m. on Tuesday, heavy rainfall in North Portland led to a combined sewer overflow (CSO) that lasted approximately 16 minutes. The overflow occurred simultaneously in two locations: near Cathedral Park on the Willamette River and near N. Upland Drive on the Columbia Slough. No other areas of the river have been affected.

As a precaution, the Bureau of Environmental Services advises the public to refrain from coming into contact with the river downstream of Cathedral Park for the next 48 hours due to the possibility of increased bacteria levels in the water. Additionally, individuals engaging in recreational activities on the Columbia Slough are advised to avoid contact with the water for the same 48-hour period.

Environmental Services also recommends that people fishing should either wait until the advisory period ends or, if exposed to river water, thoroughly wash their hands.

This CSO event occurred after a burst of rain dropped over half an inch within an hour in North Portland.

It’s important to note that combined sewer overflows are primarily composed of stormwater (about 80%) and sewage (about 20%). They are infrequent occurrences and typically happen during periods of heavy rain or snowfall. Since the completion of the Big Pipe project in 2011, the number of CSOs has significantly decreased: by 94% to the Willamette River and by more than 99% to the Columbia Slough.

This marks the third CSO incident affecting the slough in the past 23 years and the first overflow into the river since May.

Residents and concerned parties can continue to monitor the Big Pipe system, which directs rainwater and sewage to the city’s primary treatment plant.