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 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.

• If 2 or 3 samples today have bacteria concentrations above 320 bacteria/100 mL, Public Health will usually recommend closing the beach.
• If only 1 sample today is above 320 bacteria/100 mL, Public Health will also evaluate all bacteria results from the last 30 days:
  • If there is at least 1 other sample above 320 bacteria/100 mL in the last 30 days, Public Health will usually recommend closing the beach.
  • If the average of all samples (geometric mean) in the past 30 days is above 100 bacteria/100 mL, Public Health will usually recommend closing the beach.
  • Otherwise, the beach will usually remain open.
• If no samples today are above 320 bacteria/100 mL, Public Health will usually recommend that the beach remains open, or that the beach may reopen if it is currently closed. 

How to understand bacteria results:

When you are looking at the bacteria results on our website, here is how the protocol works:

• Start by looking at the three bacteria results for the day. If there are two or three high bacteria results on the same day, Public Health will generally recommend that people stay out of the water at that beach.
• If there is only one high bacteria result that day, also look at the 30-day metrics. The 30-day geometric mean is one way to average all the bacteria results from the past 30 days. If the 30-day geometric mean bacteria is above 100 bacteria/100 mL, or if there are 2 or more high bacteria samples in the past 30 days, Public Health will generally recommend that people stay out of the water at that beach.
• If there are no high bacteria results that day, Public health will generally say it is OK to swim at the beach. The 30-day metrics do not matter when there are no high bacteria results that day.

If you want to follow this protocol for your own bacteria testing, please be aware that it was designed for sampling once per week, and collecting three samples at a time. If you sample more or less than that, please contact us (lakes@kingcounty.gov) and we can help you adapt the protocol to fit your sampling and achieve a similar level of protection.

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.