Although the Big Pipe project has significantly reduce the number of combined sewer overflows, they can still happen during heavy rains

KOIN 6 NEWS - An undated photo of combined sewer overflow into the Willamette River.Heavy weekend rains have triggered combined sewer overflows into the Willamette River from multiple outfalls.

The Bureau of Environmental Service, which operates the city's sewer system, is advising people to avoid contact with the river for at least 48 hours after the overflows end because of increased bacteria in the water. There is no estimate when they will end because of the continuing rain.

A combined sewer overflow is about 80 percent stormwater and 20 percent sanitary sewage. They used to occur much more frequently, but the $1.4 billion Big Pipe project completed in 2011 reduced them by 94 percent to the Willamette River and 99 percent to the Columbia Slough.

Despite that, CSOs — as they are commonly callled — can still happen during especially heavy rains. 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.

According to BES, the Big Pipe project including 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.

For more information about CSO events, what they are and why they occur, visit