The beach at Marina Park will remain closed, possibly through Monday, officials said.
Swimmers and boaters have been asked to steer clear of the park after an estimated 68,000 gallons of sewage poured into Lake Washington for a full hour, the King County Waste Water Division said.
It was enough to cut off access to the water.
The county says the overflow started at a Kirkland pumping station a few blocks away, where transmitter malfunctions caused a cascade effect in the system and led to the leak.
The overflow was discovered during daily monitoring and maintenance of the station.
Authorities are still trying to figure out how bad the contamination is and continue to take water quality samples..
"Accidents are going to happen, but it's just really gross," one Kirkland resident said.
The Kirkland Pump Station has been operating for more than 40 years. In 2013, King County will invest $2 million in a project to increase the station's pumping capacity, replace aging equipment, and install new, larger diameter pipes, according to a county press release.
The spill comes several weeks after Seattle and King County settled a federal lawsuit over the hundreds of millions of gallons of waster water that flow into local waters each year.
They'll pay $1.4 billion in upgrades, along with a $750,000 fine.