Summer heat can cause dangerous conditions, harmful algal blooms in Idaho’s rivers and lakes

As temperatures increase, the toxins cyanobacteria produce are harmful to humans, pets, livestock and wildlife, writes guest columnist Drew Pendleton.

July 17, 2023 4:15 am
cyanobacterial bloom in water

If the water in Idaho’s rivers and lakes smells bad, looks foamy or thick (like paint was spilled into the water), appears scummy, blue-green, or brownish-red in color, stay out, health officials advise. (Courtesy of the Centers for Disease Control and Prevention)

As temperatures increase, so does the likelihood of a cyanobacterial harmful algal bloom, or cyanoHAB. CyanoHABs are caused by cyanobacteria, and the toxins they produce are harmful to humans, pets, livestock and wildlife.

CyanoHABs can change and increase rapidly if conditions are right. They favor warm water, sunlight and specific nutrients in the water.

These blooms can last throughout the summer into fall.

How to report an algal bloom in Idaho

The Idaho Department of Environmental Quality samples and analyzes Idaho water bodies for cyanoHABs. The test results are provided to the Idaho Department of Health and Welfare and local public health districts, and they decide if a health advisory should be issued.

For information on advisories, visit the DHW’s Idaho Recreational Water Advisories map for more information.

The severity of symptoms depends on how a person or animal was exposed, how long they were exposed and the toxin type.

Health effects for people

Exposure to cyanobacteria and their toxins makes people sick. People are exposed by swimming in or swallowing contaminated water and breathing in tiny droplets in the air that contain toxins. Symptoms include skin irritation or rashes, blisters, lung irritation, abdominal cramps, diarrhea and vomiting; and they can lead to liver, kidney and nervous system damage.

Health effects for pets (animals, livestock, and wildlife)

Pets are most often exposed by drinking and swallowing contaminated water and licking the toxins off their fur. The fur of animals can collect a large amount of toxin, making it very dangerous for pets. Symptoms include excess drooling, vomiting, diarrhea, foaming at the mouth, jaundice, blood in urine or dark urine, stumbling, abdominal tenderness and loss of appetite.

How to protect yourself, your family, and your pets

Not all cyanoHABs are visible, and not all bodies of water are sampled. Therefore, the public is not always notified of all blooms throughout the state or the toxins they might be producing. When planning recreational activities in any body of water in Idaho, visit the DEQ and DHW websites for information, and always use caution by doing the following:

  • If the water smells bad, looks foamy or thick (like paint was spilled into the water), appears scummy, blue-green, or brownish-red in color, stay out.
  • If an advisory is posted, keep yourself, family, and pets out of the water.

If an advisory is in place:

  • Avoid swimming, wading, or other contact with the water. Take extra care to ensure children do not drink or get the water on them.
  • Ensure pets and livestock do not drink or go into the water. If they have contact with the water, clean skin, hide, or fur with clean water right away.
  • Do not drink or cook with the water. Boiling or filtering the water does not remove the toxins and can increase the risk of becoming sick.
  • Wash hands thoroughly in clean water after handling fish or objects from the water.
  • If you choose to eat fish from the water, clean and wash fish thoroughly in uncontaminated water. Filet the fish, and remove all fat, skin, and internal organs before cooking. Cyanotoxins can build up in fish, and the risk to people is unknown.
  • Watch for symptoms. If you touch or swim in the water or breathe in water droplets, you might experience a rash, hives, red eyes, wheezing, coughing, or shortness of breath. If you swallow the water, you might have stomach pain, diarrhea or vomiting. You might have a headache, muscle weakness or dizziness. If your liver is damaged, your skin might turn yellow, and you will have dark urine. If you think you might be sick from cyanotoxin, consult your health care provider or call the poison center at 1-800-222-1222. Notify [email protected].
  • Monitor media reports and the Department of Health and Welfare’s website for health advisories.

As a general rule for anglers, swimmers, dog walkers, and anyone eager to jump into their favorite body of water, if a health advisory is in place, it should not be considered free and clear until that advisory is removed from the Idaho Recreational Water Health Advisories map.

The bottom line for Idaho’s waterways is that cyanobacteria will not immediately disappear with cold weather and will remain in the surface water for long periods of time. You are likely to see green patches of water until Idaho’s rivers and lakes ice over.

Here are more helpful resources to check out


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Drew Pendleton
Drew Pendleton

Dr. Drew Pendleton serves as state toxicologist for the Idaho Department of Health and Welfare. He was born and raised in Arlington, Texas, and went to college at Texas A&M University in College Station, where he earned his B.S. in Bioenvironmental Sciences, an MPH in Environmental Health, and a Ph.D. in toxicology. He began working for DHW in October 2021 while completing his Ph.D. studies and moved to Boise in January 2022. He serves the residents of Idaho in many ways such as health education, site assessment, community outreach, and advising the public of environmental health risks.