Heavy rains Wednesday caused Portland's combined storm and sanitary sewers to overflow, sending untreated sewage into the Willamette River.
The Portland Bureau of Environmental Services warns the public to avoid recreational activities in the river, such as water skiing, jet skiing and swimming, which could result in people swallowing river water, until 48 hours after the rain has stopped.
People who catch fish should wash their hands following contact with the water, and should cook the fish thoroughly to kill bacteria.
Combined sewer overflows are a perennial problem in Portland when heavy rains fall into the streets, overwhelming the city's sewer system.
Most of the problem is expected to subside by December, when a 20-year program to expand the city's sewer system will be completed.
Upon completion, bureau officials expect sewer overflows to occur an average of four times each winter, and once every three years during the summer, instead of the previous average of 50 times a year.