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The $1.4 billion Big Pipe project eliminated most of the overflows, but heavy rains can still ovewhelm the system.

Heavy rains caused Portland's combined sewer system to overflow into the Willamette River at several locations Sunday afternoon, according to the city Bureau of Environmental Services.

Because of increased bacteria in the water, BES says the public should avoid contact with the Willamette River for 48 hours after the overflows end. The event began at about 1:30 p.m. and was continuing through Sunday afternoon. The volume is not yet known.

The sewer pipe overflows began at the outfall near the Ross Island Bridge and extended downstream. Several other outfalls are affected.

This is the third CSO of the winter season. The previous overflows occurred on Thnaksgiving Day 2016 and Jan. 18 of this year.

Before the city completed the CSO control 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.

Portland's combined sewer system carries sewage and stormwater runoff in the same pipes. In December 2011, Portland completed a 20-year program of sewer improvements, including constructing big pipes on both sides of the Willamette River and along the Columbia Slough. The cost of the Big Pipe project was $1.4 billion.

According to BES, the improvements eliminated 99 percent of CSOs into the slough and 94 percent from the river. But, during heavy storms, the big pipes store large quantities of stormwater and sewage while pumping it to the Columbia Boulevard Wastewater Treatment Plant, and some of it can overflow.

BES says a combined sewer overflow is about 80 percent stormwater and 20 percent sewage.

For more information about CSO events, what they are and why they occur, visit www.portlandoregon.gov/bes/article/565061.

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