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Water quality monitoring at beaches

Public Health reviews the public health impact of bacteria, toxic algae, and other contaminants at beaches. We then decide the best next steps to protect people’s health, such as closing a beach.

We monitor water quality at:

  • Lake swim beaches
  • Marine beaches
  • Recreational shellfish beaches

Lake swim beaches

Bacteria monitoring

During the summer (typically May through September), Public Health reviews water test results every week for bacteria at approximately 30 popular swim beaches on Lake Washington, Lake Sammamish, Green Lake, and other lakes. 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.

Bacteria at lake swim beaches are not regularly monitored outside of the swim season. Public Health recommends that people stay out of the water for a few days after a heavy rain because rainwater tends to wash bacteria off the ground and carry them into streams and lakes.

Toxic algae monitoring

Public Health keeps an eye on toxic algae on lakes year-round, but the peak season is usually spring to fall. If there's a high risk that people could get sick from swimming or playing in the water, we recommend closing the beach or the entire lake. Once there's less risk that people will get sick, we recommend reopening the beach or the lake.

More information:

Marine beaches

During the summer (typically from Memorial Day to Labor Day), Public Health reviews water test results every week for bacteria at approximately 10 marine beaches in King County. 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.

More information:

Recreational shellfish beaches

Year-round, Public Health works with the Washington State Department of Health (DOH) to protect recreational shellfish harvesters. DOH evaluates shellfish test results for biotoxins and decides if a beach should be closed to recreational shellfish harvest if the results are unsafe. We work with city partners and others to post advisory signs at beaches to warn people not to collect shellfish.

More information: