Swimmers Beware: What To Know Before Diving In

Picture this: It’s a steamy Saturday in August. The thermometer is pushing 90 degrees. There’s no breeze where you live, the nearby park is in full sun, and it’s just too hot for the kids to play outside. Everyone is inside, trying to stay out of each other’s way. The window AC unit is cranked up, all the fans are going at full speed, but it still feels claustrophobic inside.

You know what sounds really nice right now? A refreshing dip in Lake Washington. So you get everyone ready, pack some snacks, slather on the sunscreen, and load up the car. “This is going to be great!” you think. You pull into the parking lot and it’s not as crowded as you thought it would be for such a hot summer day. “What great luck!” you say to yourself. The family piles out of the car and the kids make a beeline for the water. Except wait. One of the kids point to a sign on the beach that says no swimming. No swimming? That can’t be possible! Who would close a swimming beach on a day like today?!

Unfortunately, signs like this are a common site throughout the Puget Sound region during the summer, when harmful algal blooms (HABs) and high bacteria counts make it unsafe for people or animals to enter the water.

Harmful Algal Blooms (HABs)

two photos side by side. Left one shows an rust-colored algal bloom. Right one shows a green colored bloom.

Harmful Algal Bloom. Photo courtesy of WA Dept. of Ecology

HAB’s happen when excess nutrients, like nitrogen and phosphorus from people’s lawns and gardens, get into the water and cause algae to grow out of control. What do they look like, and what should you do if you encounter one? Generally, you will notice a thick, bluish-green mat on the water. As the bloom starts to break down, you may notice a strong odor as well. Most importantly, do not come into contact with the water, and do not let your pet come into contact with the water.

If you think you’ve found a bloom, report it to your local city or county, or the Washington State Toxic Algae Program.

close up of a "Closed No Swimming" sign at a local beach.
No swimming sign at local beach. Photo courtesy of WA Dept. of Ecology

High Bacteria Counts

Most public swimming beaches are monitored for bacteria regularly during the summer months. If bacteria is found at levels that pose a threat to people or pets, the beach will be closed to the public until the bacteria levels go down.

Where does the bacteria come from? Most often it’s from some type of poop, either from people, dogs, geese, or some other kind of wildlife. Here are a few things you can do to help:

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