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King County tests the water at many popular public swimming beaches on Lake Washington, Lake Sammamish, Green Lake, and other lakes. For a map and list of the beaches where we test the water, see the King County Lake Swimming Beach homepage.

Unfortunately, we cannot test every lake and stream where people like to swim. For more information about water quality in other lakes and streams in King County, see 'Where can I learn about the water quality in other lakes and streams?' below.

Beaches are tested once a week, usually on Monday or Tuesday afternoons. Results are available on our website later that week, usually on Wednesday afternoons.

Bacteria testing begins in mid-May and ends in mid-September. Toxic algae testing begins in early June and ends in late October.

Some beaches have different testing schedules. Those are beaches where the city contracts with King County to add the beach to our testing program. Some cities choose different start or end dates for bacteria testing. And some cities choose not to test for toxic algae every week, though at all beaches we test for toxic algae any time there is a visible algae bloom.

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 bacteria we test for is a harmless type of bacteria found in the poop of all warm-blooded animals like people, dogs, cows, geese, and ducks. We use the bacteria test results to predict the chance that people will get sick from germs that might be in the water. There are many different types of germs that can come from poop (bacteria, viruses, parasites, etc.), and it is not possible to test for each one. Instead, we test for one type of bacteria that is easy to measure and is commonly used to predict the risk of getting sick from swimming.

To learn more about sample collection or lab testing methods, full details are in the Swimming Beach Sampling and Analysis Plan. And more information about toxic algae is on the Northwest Toxic Algae website.

Public Health – Seattle & King County 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, Public Health will recommend that the park department should close the beach. They will recommend that people should not swim or go in the water for now, to avoid getting sick.

After the poop problem or algae bloom is over, Public Health reviews the water test results again. When there is a low risk of getting sick from swimming, they will recommend that the park department reopen the beach.

The Beach Closure Protocol website has more details on how Public Health determines when a beach should be closed or reopened.

If there is poop in the water, you can get sick from swimming or wading there. The most common symptoms are diarrhea (watery poop), throwing up or feeling nauseous, stomachaches, headaches, or fever. It is also possible to get infections in your eyes, ears, nose, throat, or skin. Children, elderly people, and people with weakened immune systems have a higher risk of getting sick.


If you are concerned about your health, please call your doctor and let her or him know where you were swimming.

The closure only affects the specified beach. Bacteria results can be very different over short distances (50 feet or less), so water quality in the rest of the lake is unknown.

Lake Washington and Lake Sammamish are big enough that water quality is likely much better out away from the shoreline. Historical sampling from open waters in these large lakes has generally had low bacteria. Bacteria come from poop getting into the water, usually from people, pets, or wildlife on land. In these large lakes, the poop problems from land generally do not affect the open waters of the lake. In smaller lakes, however, poop problems from land can cause high bacteria in the open waters.

Please remember to follow the safety regulations when swimming away from shore, and make sure there’s an accompanying boat that has a lifejacket for each swimmer:,

For smaller lakes also be aware of Toxic Algae Warnings that usually apply to the entire lake. For more information on toxic algae visit the Northwest Toxic Algae website.

No. When a beach is closed because of high bacteria or toxic algae, it is not safe for people or pets. Dogs often drink more lake water than most people do, so they are even more likely to get sick.

In fact, dogs should not be taken to public swimming beaches at all. Help keep poop out of the water at our beaches by taking your dog to a different place to play in the lake. Dogs are not allowed on any public beach in Seattle, and many other cities and parks also have rules that dogs are not allowed on swimming beaches.

King County tests for bacteria each month in the middle of Lake Union. Maps and data are available at the Major Lakes Monitoring website. Note that bacteria concentrations might be higher near the shore than they are in the middle of a lake, so these test results might miss poop problems at beaches or shorelines along Lake Union. For more information about swimming safety in Lake Union, read this interview with one of our King County lake scientists.

King County also tests for bacteria each month in many streams. Maps and data are available at the Streams Monitoring website. Please note that Public Health – Seattle & King County does not review these test results, and generally does not make recommendations about closures unless there is a major problem like a sewage spill.

Some older Seattle neighborhoods have combined sewers, which are pipes that carry stormwater and sewage together. The Combined Sewer Overflow website shows places where the combined sewer has overflowed recently. Avoid swimming near an overflow for 48 hours.

Algae blooms can be tested on any lake in Washington. Maps and data, as well as information on how you can report a visible algae bloom, are available at the Northwest Toxic Algae website.

If there is high bacteria in the water at a swimming beach, it very likely means that some type of poop is getting into the water. This poop could be from people, dogs, geese, or other animals.

People often assume that poop in the water comes from a sewage spill. Sewage is one possible source of poop, and we always work with local sewage utilities to investigate this. Most of the time, however, poop in the water at a swimming beach is not from sewage. Common causes of poop in the water at a swimming beach include:

  • People carry poop into the water on their bodies. To help keep poop out of the water, adults and children should wash well after using the bathroom and shower off before swimming. All babies and toddlers should wear good quality swim diapers.
  • Dogs also carry poop into the water. And if the dogs poop on or near the beach, the poop can wash into the water. This is one reason why dogs are not allowed at most public swimming beaches.
  • Geese and ducks poop on and near the beach, and the poop washes into the water. Please do not feed geese or ducks (or other wildlife) near the beach. More food attracts more birds, which leads to more poop washing into the water.
  • Streams can also carry poop from upstream areas to a swimming beach. People, pets, livestock, geese, and ducks in upstream areas can cause problems at a downstream swimming beach.


When a beach is closed because of high bacteria, we first try to find the source of the poop. We talk with the field staff who collect the water samples, and the parks staff who manage the park and the lifeguard program, to understand what has been going on at and near the beach. We also contact local sewage utilities about the possibility of a sewage spill. 

Sometimes we run additional laboratory tests that help figure out what type of animal the poop is coming from. We do this by measuring two specific strains of bacteria: one that is found only in human poop, and one that is found only in dog poop.

Once we understand more about the poop sources, we work with beach managers to help them keep poop out of the water. Every beach is unique. Here are some examples:

  • Remind people that dogs are not allowed at the swimming beach, and geese and ducks should not be fed near the beach.
  • Clean up goose poop from docks to keep it out of the water at the swimming area.
  • Reduce the number of geese near the beach, by using shiny mylar strips (“scare tape”) or specially trained dogs.
  • Renovate docks to allow more water circulation through the beach area.
  • Improve drainage in the park area near the beach, to reduce bacteria washing into the lake near the beach.
  • Reduce poop getting into streams near beaches.

Algae are a natural part of the lake ecosystem, but too much algae can cause problems. Algal blooms are caused by excess nutrients (nitrogen and phosphorus). Nitrogen and phosphorus can come from many sources such as lawn fertilizer, pet or livestock waste, erosion, or leaves and grass clippings. These nutrients can wash directly into the lake from lakeshore properties, or wash into streams that flow into the lake. Phosphorus can also build up in lake sediments and get released into the water, so past sources of phosphorus pollution can still contribute to algal blooms today.

If a lake has frequent toxic algal blooms, reducing these is a long-term project. We use water-quality data from the King County Lake Stewardship Program, the King County Stream Monitoring Program, and additional studies to help understand the sources of nutrients in the lake. We work with local residents and park managers to develop a lake-management plan and make improvements. In some cases (such as Beaver Lake, in Sammamish) local residents have created an official Lake Management District that can collect property fees to help fund lake improvements.

For more information about toxic algae, please visit the Northwest Toxic Algae website.

Still have questions?  Please contact the Science Section Lakes and Streams Team at, or call the Water and Land Resources Department Reception Desk at 206-477-4800.