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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

Beaches are sampled every week, usually on Monday afternoon. If a beach is closed due to high bacteria concentrations, it is sampled twice a week (usually on Monday afternoons and Thursday mornings) 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.
At all beaches, King County tests the water for E. coli bacteria concentrations each week. 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. To learn more about sampling or analysis methods, full details are in the Swimming Beach Sampling and Analysis Plan.
Fecal_coliform_platePhoto courtesy of DNRP

E. coli are a group of bacteria that live in the intestines of all warm-blooded animals, including people, dogs, geese, and ducks. We use the E. coli concentration to predict the risk of getting sick from germs that might be in the water. There are many different types of germs that might be found in lake water (bacteria, viruses, parasites, etc.), and it is not possible to test for each one. Instead, we test for E. coli because it is easy to measure, at it is commonly used to predict the overall risk of getting sick.

Before 2019, we measured fecal coliform bacteria instead of E. coli. Fecal coliform bacteria are a group of bacteria that includes E. coli, but also includes some other types of bacteria. Studies by the US Environmental Protection Agency have shown that E. coli is better than fecal coliform for predicting the risk of getting sick from germs that might be in the water.

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.


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.

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.

If there is a high E. coli 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 high bacteria comes from a sewage spill. Sewage is one possible cause of bacteria pollution, and we always work with local sewage utilities to investigate this. Most of the time, however, high bacteria at a swimming beach is not from sewage. Common causes of bacteria pollution include:

  • People carry bacteria into the water. To reduce the amount of bacteria 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 bacteria into the water. And if the dogs poop on or near the beach, bacteria 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 bacteria wash 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 bacteria from upstream areas to a swimming beach. People, pets, livestock, geese, and ducks in upstream areas can impact a downstream swimming beach.

When a beach is closed due to high bacteria, we first try to identify the source of bacteria. 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 bacteria are coming from. Some species of bacteria live only in human intestines, while others live only in dogs, or livestock, or geese.

Once we understand more about the bacteria sources, we work with beach managers to help them reduce bacteria concentrations. 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 bacteria 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.