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A 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 2019) are described below.

Bacteria closures

King County measures bacteria concentrations at the swimming beaches each week, usually on Tuesdays. We collect three water samples from different parts of the beach, and average the results together (using the arithmetic mean) to get the average bacteria value for that day. 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 values are high, Public Health – Seattle & King County will review the monitoring data and other information about the beach, and recommend that beach managers close the beach to public use. 

A beach is usually closed if one or both of these criteria are met:

  • A day’s average bacteria value is over 1000 CFU/100 mL, or
  • The average of the bacteria values for the three most recent sampling days is over 200 CFU/100 mL. The three-day average is calculated using the geometric mean (geomean).

A beach is usually reopened if both of these criteria are met:

  • The three-event average (geomean) bacteria value is below 200 CFU/100 mL, and
  • The daily average bacteria values from the two most recent sampling days are both below 200 CFU/100 mL.

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. If samples are not collected in two consecutive weeks, the two most recent samples may be used depending on specific lake conditions. 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