Leer en español | Cross-posted on Headwaters

There’s nothing quite like relaxing on a beach by the lake in the summertime. Here in King County, we’re lucky to have dozens of great beaches to choose from, and the Lake Swimming Beach Program works hard to make sure the water is safe for people to swim.

Every week during the summer (typically from mid-May through mid-September) we test the water at popular swim beaches in King County for bacteria. If there’s a high risk that people could get sick from swimming or playing in the water, we recommend closing the beach. Once there’s less risk that people will get sick, we recommend reopening the beach.

We get lots of questions about beach water quality testing. See below for the answers to some of the top questions!

Which beaches do you test water quality at?

We test for bacteria at approximately 30 popular swim beaches on Lake Washington, Lake Sammamish, Green Lake, and other lakes.

You can see the full list of beaches on the swim beach page: https://kingcounty.gov/swimbeach.

How do you decide when to close a beach?

Public Health reviews the water test results for bacteria and toxic algae to predict the risk of getting sick from swimming or playing in the water. When there is a high risk of people getting sick, we recommend that people should not swim or go in the water for now, to avoid getting sick. Learn more about our testing protocol.

You can sign up to get email or text alerts about beach closures and reopenings in King County. You can also check your city’s social media accounts for announcements. For City of Seattle beaches, announcements are posted by Seattle Parks and Recreation (Follow Seattle Parks & Recreation on Instagram and Facebook).

What kind of bacteria do you look for?

We’re looking for bacteria that tells us if there’s poop in the water. This poop usually comes from wildlife, like geese, and it carries germs that can make people sick. Generally, beach closures during the summertime happen because of bacteria from wildlife poop, not because of sewage overflows.

One big way to help keep our lakes healthy is to not feed the birds!

What happens if someone goes into the water during a closure?

When people swim in the water while the bacteria levels are high, people are at a higher risk of getting sick. Illnesses can include stomach issues, like nausea, stomach cramps, and diarrhea. It can also include eye infections and skin rashes.

Not everyone who enters the water at a closed beach will get sick – it depends on how high the bacteria levels are, the health status of the person getting in the water, and how much time they spend in the water. Kids, pregnant people, older adults, and people with compromised immune systems are at the greatest risk.

The bottom line: Stay out of the water at a closed beach to help avoid illness.

Are beach closures increasing each summer?

Not necessarily. Some summers have more closures and others have fewer.

It’s important to note that in 2023, we changed the criteria we use for these closure decisions to align with current scientific guidelines. As a result of this change, we issue more frequent beach openings and closures, and the beach closures last for a shorter duration of time.  

Our new criteria are based more on the real-time water quality data that we get each week. So, when water quality gets worse at a beach, it’ll close more quickly. When water quality improves, the beach will open faster.

How can water quality be bad at one beach but okay at the next beach over?

Overall, beaches on Lake Washington have great water quality and low bacteria levels. The bacteria problems that close a swimming beach are usually small-scale issues from poop (usually goose poop) right on or near the beach that washes into the water. The bacteria levels can be much lower at a neighboring beach with fewer poop problems.

Know before you go! Check if a beach is open at https://kingcounty.gov/swimbeach.

Originally posted May 21, 2026.