Looking to go for a cool dip in one of King County’s lakes? Before you head to the beach, check the water quality and temperatures using the interactive map below, which we will be updating for the rest of the summer. 

Every week from mid-May to early September, the King County Lake Swimming Beach Program measures bacteria concentrations at selected high-use public beaches. Most of these beaches are on Lake Washington or Lake Sammamish, but others are on Green Lake, Echo Lake, Beaver Lake, Pine Lake, Lake Wilderness and Rattlesnake Lake.

Bacteria is an indicator of fecal matter, usually from humans, pets or wildlife on land, getting in the water.  

Among the 33 freshwater beaches where water bacteria levels are being monitored, 17 have logged at least one week of high or moderate levels of concern this summer. Swimming is still safe in bacteria levels of moderate concern, according to the King County Department of Natural Resources and Parks, unless it is persistent in three consecutive samplings, which at that point may lead to a closure.

“There aren’t simple recommendations — bacteria concentrations are highly variable, so one day in that ‘medium’ range might just be a fluke, rather than meaningful information,” said Daniel Nidzgorski, an ecologist at DNRP.

“I’d trust Public Health to recommend closing the beach if and when they do get unsafe.”


As of Sept. 2, the following beaches are closed due to high bacteria levels: Meydenbauer Bay Beach and Newcastle Beach.

The program has issued a toxic algae notice for Lake Wilderness in Maple Valley.

Visual reporting of local news and trends is partially underwritten by Microsoft Philanthropies. The Seattle Times maintains editorial control over this and all its coverage.

The count of bacteria is measured as colony-forming units (CFU) in about half a cup of water (per 100 mL). 

To measure the level of bacteria at a beach, county officials collect three water samples from different areas of a beach and average the results to get the average bacteria value for the day. The Public Health — Seattle & King County department then reviews this data and other information to recommend closures to the beach manager. 

The closure affects only the specified beach — not the entire lake — as bacteria results vary even over short distances of 50 feet or less. 


Typically, a beach is closed when the sampling day’s average bacteria measures over 1000 CFU/100mL, or if the average bacteria value for the three most recent sampling days is over 200 CFU/100 mL. This three-day geometric mean is how the county measures moderate-but-persistent poop problems. 

A closed beach may reopen when the average bacteria is below 200 CFU/100 mL in three consecutive samplings, and the daily average of the two most recent sampling days was below 200 CFU/100 mL. 

“Once the poop source is fixed and there’s no new poop getting into the water, the bacteria that’s already in the water naturally washes away or dies off pretty quickly,” Nidzgorski said. Public Health, he said, doesn’t recommend reopening a beach until they have evidence bacteria concentrations are consistently staying low.

Lake Washington and Lake Sammamish are big enough that water quality is likely much better out away from the shoreline in the open waters, the Department of Natural Resources and Parks said. In smaller lakes, however, fecal matter from land can cause high bacteria even in open waters. Be sure to follow safety guidelines when swimming away from the shore.

How to help keep beaches safe and open

Unlike pools, the water at lake beaches does not get treated.

Here’s how the public can help keep poop out of water and help beaches stay safe and open:

  • Don’t feed geese or ducks near the beach. “Food attracts them, and more geese and ducks means more poop at the beach,” Nidzgorski said.
  • Don’t bring your dog to the swimming beach. Most swimming beaches do not allow dogs.
  • Use a good swim diaper for babies and toddlers to keep poop well-contained. 
  • Shower before going in the water. “Everybody has a little bit of poop on our bodies,” Nidzgorski said. “When you get 3,000 people a week swimming at a popular beach, all those little bits really add up.”

What to do where the water is not tested 

Since the King County Lake Swimming Beach Program does not test the water quality at every freshwater swimming hole, Nidzgorski recommends scoping the area around the shoreline before getting into the water.

Look and smell around the shoreline and look for signs of toxic algae, he said. Proceed with caution if you see or smell poop from people, pets or wildlife. 

He also recommends waiting a day or two after it rains before getting into the water. “During summer dry weather, poop builds up on land. And then when it rains, all that poop can wash into the water at once.”