CSO Advisory: Heavy rains cause combined sewage and stormwater to overflow to the Willamette River, avoid contact for 48 hours

News Article

Update: The overflow  that began Sunday around 9:15 a.m. lasted about 2.5 hours and released an estimated 25 million gallons of combined stormwater and sewage to the Willamette River from seven outfalls downstream of the Ross Island Bridge. The public was advised to avoid contact with the Willamette River for 48 hours, that time period lasted through noon today.  

Heavy rains caused Portland’s combined sewer system to overflow to the Willamette River around 9:15 a.m. this morning from several outfalls.

Because of increased bacteria in the water, the public is advised to avoid contact with the river downstream of the Ross Island Bridge for 48 hours after the event ends. The overflows are still continuing. The volume is not yet known. 

Combined sewer overflows (CSOs) are rare and occur during periods of heavy rain or snowfall. A CSO is about 80 percent stormwater and 20 percent sewage. 

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 first overflow of 2018 and the first since October 22.

Find out more information about CSO events, what they are and why they occur.


Diane Dulken

Public Information Officer