Portland's Big Pipe project

Heavy rainfall on Wednesday prompted a combined sewer overflow, city officials said, despite the big pipe project (Stuart Tomlinson/The Oregonian)

Portland's first significant storm of the fall season triggered a massive sewer overflow into the Willamette River on Wednesday.

The rainfall produced a combined sewer overflow warning from the Bureau of Environmental Services.

City officials said the public should avoid contact with the Willamette River for at least the next 48 hours as untreated sewage will likely continue to flow into the river until rainfall tapers off.

Wednesday's sewage overflow is the third such event in 2014, according to BES spokesman Linc Mann, and the 10th since the bureau finished the $1.4 billion Big Pipe project in 2011.

"A big rain like this is going to cause it," Mann said Wednesday.

Eight sewer outfalls in the city's extensive system are all overflowing as of Wednesday at 4:50 p.m., according to Mann.

"It is especially important to avoid recreational activities, such as jet skiing or swimming, during which water could be swallowed," Mann wrote in a press release.

The city issues the CSO warnings as a matter of public health. Prior to the Big Pipe's completion, Portland averaged 50 sewer overflows into the Willamette River each year.

Wednesday's warning applies to the river from south of the Sellwood Bridge all the way to the confluence of the Willamette and Columbia Rivers.

Residents can check the status of overflow warnings online or by calling 503-823-2479.

-- Andrew Theen