Skip to Main Content View Text-Only

The City of Portland, Oregon

Environmental Services

working for clean rivers

Phone: 503-823-7740

Fax: 503-823-6995

1120 SW 5th Ave, Suite 613, Portland, OR 97204

More Contact Info

Subscribe to RSS feed

Most Recent

View Less

UPDATED CSO Advisory: Heavy rains lead to combined sewer overflow to Willamette River

UPDATE: The overflow that began at 1:15 am ended at 5:30 am. The advisory remains in effect for 48 hours. Avoid direct contact with Willamette River until Tuesday morning due to increased bacteria in the water. 

(October 22, 2017) -  The "atmospheric river" passing overhead through Portland this weekend has affected the Willamette River. Heavy rain yesterday and overnight  caused Portland’s combined sewer system to overflow to the Willamette River around 1:15 a.m. this morning from multiple outfalls.  

Because of increased bacteria in the water, the public is advised to avoid contact with the Willamette River for 48 hours after the CSO event ends. The overflows are still continuing. The volume is not yet known. A combined sewer overflow is about 80% stormwater and 20% sanitary sewage.

According to the National Weather Service, more than two inches fell in Portland on Saturday and the storm system called an atmospheric river may continue to bring periods ofrain to the Portland area through Sunday. 

CSOs are rare and occur during periods of extreme 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 been reduced 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 residences 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.  

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

The Bureau of Environmental Services provides city residents with programs to protect water quality and public health, including wastewater collection and treatment, sewer construction and maintenance, stormwater management, and stream and watershed restoration. Visit and follow us on Twitter for news @besportland.

Media contact: Diane Dulken 503.823.6724