Skip to main content
Our website is changing! Starting March 31, 2023 our website will look different, but we're working hard to make sure you can still find what you need.  
King County logo
Newprt-kidsPhoto courtesy of Jonathan Frodge

What does the program do? 

Every week from mid-May through mid-September, we test the water at many popular public swimming beaches on Lake Washington, Lake Sammamish, Green Lake, and other lakes. We test for bacteria, which tells us whether there is poop in the water from people, pets, or wildlife. And at many beaches, we also test for toxic algae. Poop or toxic algae can make people sick from swimming or playing in the water.

The King County Environmental Lab collects and tests more than 1,600 water samples every year from the beaches. Public Health – Seattle & King County reviews the water test results to see if there is too high a chance of people getting sick from swimming or playing in the water.

In any lake, stream, or the Puget Sound, there is always some chance of getting sick from swimming or playing in the water. That’s why it’s always a good idea to wash your hands after you’ve been in the water or playing in the sand, especially before you eat. Most of the time, there’s a low chance of getting sick. But any time that there’s too high a chance of getting sick, Public Health will recommend that people stay out of the water for now.

What is the history of the program?

Fecal coliform platePhoto courtesy of DNRP

This program started in 1996 testing the water at 15 beaches on Lake Washington, Lake Sammamish, and Green Lake. The program has grown over time, and in 2022 tested the water at 27 beaches on lakes throughout King County. Water testing at most of the beaches is funded by the King County Wastewater Treatment Division, and water testing at other beaches is funded by the city that manages that beach.

From 1996 to 2004, we tested the water for bacteria only. In 2005, we started testing for toxic algae as well, using new methods developed by the King County Environmental Lab. In 2019, we changed what type of bacteria we test for. From 1996 to 2018, we tested for a group of bacteria called fecal coliform bacteria. But now we test for a more specific type of bacteria (within the fecal coliform group) called E. coli. These are harmless bacteria found in the poop of all warm-blooded animals like people, dogs, cows, geese, or ducks. Studies by the US Environmental Protection Agency showed that E. coli is better than fecal coliform for predicting the chance of people getting sick from swimming.