A power malfunction at an Everett sewer lift station resulted in sewage being released into Port Gardner Bay on Saturday night, the Everett Fire Department said.
Power to the lift station was lost at about 10 p.m. and was restored by about 11 p.m.
The city is working to determine the quantity of the spill and water quality samples have been taken.
As a precaution, signs notifying people to stay out of the water have been posted at Howarth Park and Pigeon Creek beaches.
The city has notified the Snohomish County Health District and the Washington State Departments of Health and Ecology of the incident.