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lake swimming beach can be closed if there is a high concentration of either bacteria or algal toxins in the water. The current closure and reopening protocols (revised in 2023) are described below.

Bacteria closures

King County measures bacteria concentrations at the swimming beaches each week. We collect three water samples from different parts of the beach. Bacteria are measured as colony forming units (CFU), which is a count of the number of bacteria, per 100 mL of water (about a half-cup of water).

If the bacteria concentrations are high, Public Health – Seattle & King County will review the monitoring data and other information about the beach, and may recommend that beach managers close the beach to public use.

2023 closure protocols coming soon!

Algal toxin closures

Some beaches are tested for algal toxins every week, while other beaches are tested only if there is a visible algal bloom. If one or more algal toxins at a beach are above the following Washington State Department of Health Recommended Guidance thresholds, Public Health will usually recommend closing the beach:

  • Microcystin: 8 µg/L
  • Anatoxin-a: 1 µg/L
  • Saxitoxin: 75 µg/L
  • Cylindrospermopsin: 15 µg/L 

To reopen a beach, algal toxins need to be below the state guidance thresholds for two consecutive weeks. Algal blooms can start producing toxins quickly, so Public Health will often recommend keeping a beach closed if there is still a visible algal bloom, even if the toxin concentrations periodically drop below the threshold. 

For more information about algal toxins, visit the Northwest Toxic Algae website