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Portland officials urge residents to avoid contact with the river until 48 hours after the overflows end.

Heavy rains caused Portland's combined sewer system to overflow into the Willamette River at several locations beginning at noon on Wednesday.

Officials with the Bureau of Environmental Services said that, because of increased bacteria in the water, the public should avoid contact with the Willamette River for 48 hours after the CSO event ends.

According to BES, this is the fifth CSO event this 2016-2017 winter season. Previous overflows this season occurred on February 9 for 13 minutes due to a debris blockage in an outfall that quickly cleared itself; on Super Bowl Sunday, February 5, due to unusually heavy rains that dropped about 2.5 inches within 24 hours; on January 18 due to a combination of snowmelt and heavy rain; and Thanksgiving Day due to heavy rain.

Portland sewer ratepayers spent $1.4 billion on a Big Pipe program intended to reduce such overflows that was completed in December 2011. Since then, the city has eliminated 94 percent of overflows to the Willamette River and 99 percent from the Columbia Slough.

Before completing the program, combined sewers overflowed an average of 50 times a year. Today, the combined system overflows to the Willamette River an average of four times per winter and once every three summers.

During heavy storms, the big pipes store large quantities of stormwater and sewage while pumping it to the Columbia Boulevard Wastewater Treatment Plant in North Portland. During very heavy storms, like the one Wednesday, some combined sewage can overflow into the river. A combined sewer overflow is about 80 percent stormwater and 20 percent sanitary sewage.

You can find out more about CSOs, what they are and why they occur at:

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