Harmful Algal Blooms Confirmed on Hebgen Reservoir- Advisory Issued

Bozeman, Mont. – July 24, 2026 – Harmful algal blooms (HABs) are present on Hebgen Reservoir in the Corey Springs area of the Grayling Arm. Routine monitoring of Hebgen has confirmed the presence of anatoxin-a which poses 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 toxins. Ingestion or prolonged contact with the algal bloom may result in illness, with impacts such as muscle twitching, staggering, convulsions, paralysis, and death.

Children and pets are more likely to ingest HAB infested water.

Toxin exposure can occur in humans from recreational activities where water might be ingested such as swimming, wind surfing, jet skiing, and water skiing. Children and pets are more likely to ingest HAB infested water 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.

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 will be posted at public access points in the Corey Spring area of the Grayling Arm 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 1-800-222-1222and seek medical attention.

Report a suspected HAB at www.hab.mt.gov 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. The Gallatin City-County Health Department has resources for water quality on their website, www.healthygallatin.org.