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Harmful algae blooms detected on Hebgen Reservoir, advisory issued

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The blue-green algae accumulates on the bank of the Rainbow Point Campground on Hebgen Lake. PHOTO COURTESY OF NORTHWESTERN ENERGY


WEST YELLOWSTONE – Harmful algae blooms are present on Hebgen Reservoir in the northeast portion, in the Grayling Arm area, of the reservoir. Routine monitoring and field testing in the Rainbow Point area of Hebgen Reservoir has detected the presence of an algal toxin, anatoxin-a, that poses a risk to people, pets and livestock. 

Ingestion or prolonged contact with the algal bloom may result in illness, with signs such as muscle twitching, staggering, convulsions, paralysis and death. Importantly, children and pets are more likely to ingest HAB infested waters because they spend most of their time wading in the shallow waters where algae can accumulate, and they have less control over how much water they ingest. Animals and livestock that drink large amounts of contaminated water, and pets that collect scum on their fur and then ingest it by licking, are at high risk of toxin exposure. Toxin exposure can occur in humans from recreational activities where water might be ingested such as swimming, windsurfing, jet skiing and water skiing. 

Health experts recommend people not swim or take part in activities likely to result in exposure to the toxin in areas where the algal bloom is present. Additionally, pets and livestock should be prevented from entering the water in that area.

HABs are caused by blue-green algae that are native to Montana’s freshwater lakes and reservoirs. Not all varieties of blue-green algae are harmful, but some can produce dangerous toxins. Bluegreen algal blooms often look like pea soup, grass clippings or green latex paint. The algae are usually suspended in the water or appear as floating mats.

Advisory signs are at public access points in the Rainbow Point area warning the public that toxic algae has been identified in the water in these areas. At this time there are no other identified blooms affecting other areas of the reservoir. NorthWestern Energy is monitoring the blooms and will provide updates if additional restrictions are implemented for public safety. 

Suspect a HAB? When in doubt, stay out. Do not drink, swallow, or swim in water that shows signs of a HAB and be sure to keep kids, pets and livestock out too. 

If you suspect a HAB-related illness in a person or animal, call Poison Control 1-800-222-1222 and seek medical attention. Report a suspected HAB at or call 1-888-849-2938. You may also report a suspected HAB by calling the Gallatin City-County Health Department Environmental Health at 406- 582-3120.

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