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

Press Release

January 29, 2024 Update: This advisory has expired.

Heavy rains led the Big Pipe system to reach capacity around 3:30 a.m. on Saturday, January 27, leading to a combination of stormwater and sewage overflowing to the Willamette River from multiple outfalls. 

The combined sewer overflow (CSO) affects the stretch of the river downstream of the Ross Island Bridge. However, the advisory may be adjusted to include areas further upstream as more rain is expected. The overflow is ongoing at this time.

During an overflow and 48 hours after an overflow stops, the public is advised to avoid contact with the river because of increased bacteria in the water. The river’s water quality is safe for recreation during all other times.

A CSO is about 80 percent stormwater and 20 percent sewage. CSOs are rare and can occur during periods of heavy rain or snowfall. The public can follow the duration of this overflow by viewing the Big Pipe Tracker online.

Since completing the Big Pipe project in 2011, the 20-year $1.4 billion program to reduce overflows, the number of CSOs has dropped by 94 percent to the Willamette River and 99 percent to the Columbia Slough. Before the project, incidents occurred to the Willamette River an average of 50 times a year, with some lasting days. Today, overflows occur an average of four times per winter season and once every three summers. 

This is the first CSO of the calendar year, and there is no immediate estimate for today’s overflow. 

The Big Pipe system refers to 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 City’s main wastewater treatment plant in North Portland. Visit About CSOs to find out more.

About Environmental Services

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.