Lake Swimming Beach Program Frequently Asked Questions (2022)

Which beaches are monitored?

King County monitors water quality at public, lifeguarded swimming beaches. Most of the beaches monitored are on Lake Washington or Lake Sammamish, plus beaches on several smaller lakes. For a map and list of monitored beaches, see the King County Swimming Beach homepage.

Unfortunately, it is not feasible to monitor every lake and stream throughout the county where people swim. This monitoring program focuses on high-use, lifeguarded beaches because they are used by the greatest number of people, especially young children who have the highest risk of getting sick.

The beaches in this program were selected to cover a wide geographic area in Lake Washington, Lake Sammamish, and Green Lake, to better understand overall beach water quality in these lakes. Some cities contract with King County to have additional beaches monitored, and some cities monitor beaches themselves and have the data posted on the Swimming Beach website.

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

When are beaches sampled?

Beaches are sampled every week, usually on Tuesday afternoon. If a beach is closed due to high bacteria concentrations, it is sampled once a week until the beach reopens. Results are available online one to two days after sampling.

Bacteria sampling begins in mid-May and ends in early September. Algal toxin sampling begins in early June and ends in late October.

Cities that contract with King County can choose to have beaches sampled less frequently. Some cities choose to have beaches sampled every other week, and some choose different start or end dates.

What do you test the water for?

At all beaches, King County tests the water for bacteria concentrations each week.Bacteria is an indicator of poop in the water.At many beaches, we also test the water for algal toxins (microcystin and anatoxin) each week. At some beaches, though, we test the water for algal toxins only if there is a visible algal bloom.

We test for a harmless type of bacteria that live in the intestines of all warm-blooded animals, including people, dogs, geese, and ducks. We use the bacteria concentration to predict the risk of getting 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 group of bacteria that is easy to measure and is commonly used to predict the overall risk of getting sick.

To learn more about sampling or analysis methods, full details are in the Swimming Beach Sampling and Analysis Plan.

How do you decide whether a beach should be closed or reopened?

If there are high concentrations of bacteria or algal toxins, Public Health – Seattle & King County uses the monitoring data to make a closure recommendation to the city or county park department that manages the beach.

Once water quality improves, Public Health uses the monitoring data to make a reopening recommendation to the park department.

The Beach Closure Protocol website has more information on the protocols used to determine when a beach should be closed or reopened.

What if I was swimming at a closed beach?

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.

When a beach is closed, can I still swim in other parts of the lake?

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:

If a beach is closed, can my dog swim there?

No. When a beach is closed due to 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. 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.

Where can I learn about the water quality in other lakes and streams?

King County monitors bacteria each month in the middle of Lake Union. Maps and data are available at the Major Lakes Monitoring website. King County also monitors 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 monitoring 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.

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

How does poop get into the water?

If there is a high bacteria concentration in the water at a swimming beach, it very likely means that some type of poop is getting into the water. This 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. To reduce the amount of poop carried into the water, adults and children should wash well after using the bathroom. 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 designated swimming beaches.
  • Geese and ducks poop on and near the beach, and the poop washes into the water. Swimming beaches often have open grassy areas with an open shoreline, an ideal environment for geese and ducks. Do not feed geese or ducks. Feeding them attracts them to the beach area, which increases the amount of 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 impact a downstream swimming beach.

What do you do to fix a poop problem?

When a beach is closed due to high bacteria levels, we first try to identify the source of the poop. We talk with the field staff who sample the beaches, 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 spill.

We may also run a set of laboratory analyses that help determine the type of animal the poop is coming from. We do this by measuring two specific strains of bacteria: one that lives only in human intestines, and one that lives only in dog intestines.

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.

What do you do to fix a toxic algae problem?

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.

Who do I contact if I still have questions?

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.

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