About 23,000 gallons of stormwater and sewage flowed into the Willamette River on Thursday evening, according to Portland's Bureau of Environmental Services.

Heavy rains caused the combined sewer overflow from a pump station at Southeast Alder Street and Water Avenue, the bureau said in a news release. The overflow started at 7:32 p.m. and stopped 14 minutes later.

Such overflows are about 80 percent stormwater and 20 percent sewage, the bureau said. The bureau advises people to avoid contact with the river from the Morrison Bridge to the Columbia River confluence for about two days because of an uptick in bacteria in the water.

The pump station, which will soon be upgraded, also overflowed for four minutes last month, the bureau said.

"The pump station, built in 1952, is scheduled to be taken offline later this month for two years of construction," the bureau said. "The upgrades will improve reliability and increase pumping capacity to prevent sewage releases into buildings and streets as well as overflows to the river."

Stormwater runoff and sewage flow through the same pipes, according to the bureau. And during periods of heavy rain, big pipes store stormwater and sewage while pumping it to a wastewater treatment plant.

By design, some stormwater and sewage can overflow during especially heavy storms.

Portland completed a $1.4 billion Big Pipe project to cut down on combined sewer overflows in 2011. The bureau said there were an average of 50 overflows into the Willamette River each year before the project was finished.

— The Oregonian/OregonLive