Partying At A Lake This Memorial Day? Look Out For Toxic Algal Blooms

If you're on your way to a lake to celebrate Memorial Day weekend, be careful of harmful algal blooms. A number of them have been detected in lakes throughout California, including popular destinations like Lake Hemet.

“Stay away from anything that looks discolored or any scum or anything that washed up on the shoreline. We recommend you keep pets away from it," said Carly Nilson, freshwater and estuarine harmful algal bloom program manager for the State Water Resources Control Board.

Algal blooms occur year round throughout the state, but tend to show up between April and May as waters warm and populations of cyanobacteria explode. There are various types of cyanobacteria, but it's usually microcystis that's responsible for the intense green water seen during some blooms. The cyanotoxins released can cause a number of health issues, from skin irritation to gastrointestinal distress, and can damage your kidneys and liver.

A boat motors through green water during a harmful algal bloom (HAB) of blue-green algae at Lake Elsinore on Aug. 25, 2022 in Lake Elsinore. Conditions aren't nearly as bad in 2024.

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David McNew

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Wet years can help prevent blooms by providing an influx of fresh, cool water into lakes and streams. However, climate change is believed to be making the problem worse, as blue-green algae seem to love hotter water temperatures, elevated carbon dioxide levels and higher nutrient loads associated with decreased runoff.

In an aerial view, green water is seen during a harmful algal bloom at Lake Elsinore on Aug. 25, 2022.

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David McNew

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Getty Images

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Lakes of concern

According to the state, you should proceed with caution at:

• Puddingstone Reservoir in Los Angeles County
• Lake Elsinore in Riverside County
• Lake Henshaw in San Diego County

And you should stay off and out of the water completely at:

• Lake Hemet in Riverside County

Look out for telltale signs of potential algal blooms.

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State Water Resources Control Board

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The state's testing a number of lakes prior to major holidays, including Memorial Day, July 4 and Labor Day. While they regularly update a map detailing algal blooms throughout the state (see below), they don't always have the most up to date information on each body of water.

“We don’t know the extent of the problem unless we’re actually evaluating it. And unfortunately we don’t have enough money to actually do a comprehensive monitoring program for [harmful algal blooms]," said Nilson.

So, don't rely on the map to tell you if a lake is safe.

“If we hear about a bloom, we’ll go out and do some followup monitoring. But if we don’t hear about it and the public’s not informing us then we don’t have the resources to proactively go out and see what the current conditions are,” Nilson said.

To that end, if you see an algal mat floating by, let the state know.

You can call 1-800-222-1222 or e-mail them at CyanoHAB.Reports@waterboards.ca.gov.