Heavy rains led to a combined sewage overflow Saturday from a single outfall at NW 110th Avenue and NW Front Avenue to the Willamette River in Portland.
The overflow began at approximately 4:46 p.m. and ended around 5:09 p.m. on Saturday. A preliminary estimate of the overflow amount is 60,000 gallons.
Because of increased bacteria in the water, the public should avoid contact with the Willamette River in the vicinity of Linnton and Port of Portland Terminal 4 and downstream for the next 48 hours.
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. This is the first overflow in 2019.
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.