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Toxic Algae in Yosemite

Recent testing yielded positive results for small amounts of toxic algae in Tenaya Creek in Yosemite Valley, and toxic algae may exist in other sites in Yosemite Valley. Toxins are concentrated within the algal mats themselves and released episodically into the water when the algae dies or is disturbed.

What does this mean for me?

  • For your safety, do not enter or drink from Tenaya Creek, which flows from above Mirror Lake to where it enters the Merced River near North Pines Campground.

  • Filtering and/or boiling the water is not effective against this type of algae.

  • Prevent pets from drinking the water and eating or touching algae. In particular, prevent dogs from eating dried algal mats on shore.

  • Please report any large algal blooms and/or algae that is particularly bright, bubbly, strange-looking, or appears like a haze in the water.

  • Do not disturb algal mats in any way. Wading or swimming can cause toxins to be released into the water.

  • If you suspect a site has toxic algae, do not enter the water and do not drink water from the area. While some sites are signed based on testing results, it’s likely that algae exists in other parts of Yosemite Valley. Don’t rely on signage alone.

Can I still swim in the river? Can I still filter or treat the water for drinking? Water that is clear with no visible algae in the area presents a low risk. Even in areas with no visible algae, watch for isolated clumps of algae floating by.

If you think algae may be in the water:

  • Do not enter the water.

  • Do not drink the water, even if treated.

  • Do not let pets into the water, allow them to drink the water, or eat algae on the shore.

What are the signs and symptoms of exposure to toxins from algae? According to the California Water Quality Monitoring Council, the following signs and symptoms may occur within 48 hours of exposure to a waterbody with a suspected or confirmed algal bloom:

  • sore throat or congestion;

  • coughing, wheezing, or difficulty breathing;

  • red, or itchy skin, or a rash;

  • skin blisters or hives;

  • earache or irritated eyes;

  • diarrhea or vomiting;

  • agitation;

  • headache; and/or,

  • abdominal pain.

If people show symptoms of cyanotoxin and/or cyanobacteria exposure after contact with water, or with scums or mats of algae, they should receive immediate medical attention. Additional resources are available from the Centers for Disease Control and Prevention (CDC), and by contacting the California Poison Control Center (1-800-222-1222). See the HAB-related Illness Tracking webpage for information on previously reported human illnesses related to HABs in California.

Is the park monitoring for harmful algal blooms? The park continues monitoring for toxic algae and testing for toxins throughout the park. You can learn more about harmful algal blooms at

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