CSO Advisory: Heavy rains Saturday led to combined sewer overflow (CSO) at St. Johns Bridge

News Article
Simplified map of Willamette River through Portland. The City's combined sewer outfalls marked with yellow dots. There 15 total outfalls: 3 near St. Johns, 4 just north of Fremont Bridge, 5 between Fremont and Ross Island bridges, 1 near south tip of Ross Island, and 2 near Sellwood.

Heavy rains led to a combined sewage overflow late Saturday night from a single outfall near the St. Johns Bridge to the Willamette River.

While Portland experienced heavy rains throughout the city this weekend, rain fall was especially intense in St. Johns. The overflow was brief; the volume is unknown at this time.

Because of increased bacteria in the water, the public should avoid contact with the Willamette River at Cathedral Park and downstream of the St. Johns Bridge through midnight tonight.

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 overflow in 2018.


Diane Dulken

Public Information Officer