Heavy rains led to a combined sewage overflow from a maintenance hole at S Macadam Avenue and S Sweeney Street to the Willamette River in Portland on Monday, September 27.
The overflow began at approximately 4:19 p.m. and stopped at about 4:39 p.m. The volume of the overflow is unknown at this time.
As a precaution, people should avoid contact with the Willamette River in that area for at least 48 hours after the discharge has ended because of the possibility of increased bacteria in the 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. Since completing the Big Pipe project in 2011, a 20-year $1.4 billion program to reduce overflows, the number of CSOs have dropped by 94 percent to the Willamette River and 99 percent to the Columbia Slough.
The Big Pipe project constructed a series of improvements, from disconnecting downspouts on homes to allow rainwater to be absorbed naturally in the ground to the construction of big pipes on both sides of the river and along the slough to store and convey large quantities of flows to the Columbia Boulevard Wastewater Treatment Plant.
Before the project, CSOs occurred to the Willamette River from multiple outfalls an average of 50 times a year, with some instances lasting days. Today, overflows occur an average of four times per winter season, and once every three summers.
This is the second CSO since January 2021.
Find out more information about CSO events, what they are and why they occur at https://www.portland.gov/bes/about-csos.