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Advisory issued for harmful algal blooms on Hebgen Reservoir

algal bloom hebgen lake.jpg
Posted at 10:05 AM, Jul 28, 2021
and last updated 2021-07-28 12:45:51-04

BOZEMAN — An advisory has been issued due to harmful algal blooms (HAB) that have been confirmed on Hebgen Reservoir.

According to a press release, HABs are present in the northeast portion, in the Grayling Arm area, of the reservoir.

The release said routine monitoring confirmed the presence of anatoxin-a and microcystin, toxins that pose a risk to people, pets, and livestock.

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 cyanotoxins. Blue-green 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 Grayling Arm 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.

Health experts recommend people not swim or take part in activities likely to result in exposure to the HAB. Pets and livestock should also be prevented from entering the water in the area.

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, wind surfing, jet skiing, and water skiing.

If you're in doubt or suspect the presence of a HAB, stay out of the water. Do not drink, swallow, or swim in water that shows signs of a HAB, and keep kids, pets, and livestock out as well. 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.

NorthWestern Energy is monitoring the blooms and will provide updates if additional restrictions are implemented for public safety.