CLOSURE - Harmful Algal Blooms Confirmed on Hebgen Reservoir

Bozeman, Mont. - July 30, 2026 - A harmful algal bloom (HAB) has been detected in the Rainbow Point area of Hebgen Reservoir, and that area is closed for water-based activities because toxin levels are high. Routine monitoring of Hebgen shows the water contains anatoxin-a, a toxin made by certain algae that can make people, pets, and livestock very sick. Other parts of the reservoir are also being monitored, but those areas currently have lower toxin levels and remain open.

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.

Closure signs will be posted 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, Northwestern Energy continues to monitor a bloom at Corey Springs which has tested positive for anatoxin-a, but at a lower concentration warranting warnings rather than full closure. No blooms have been observed at Savage Bay, but additional observation is taking place because of the proximity to the Rainbow Point bloom.

NorthWestern Energy is monitoring these 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 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.

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NorthWestern Energy, provides essential energy infrastructure and valuable services that enrich lives and empower communities while serving as long-term partners to our customers and communities. We work to deliver safe, reliable, and innovative energy solutions that create value for customers, communities, employees, and investors. We do this by providing low-cost and reliable service performed by highly-adaptable and skilled employees. We provide electricity and / or natural gas to approximately 850,300 customers in Montana, South Dakota, Nebraska, and Yellowstone National Park. Our operations in Montana and Yellowstone National Park are conducted through our subsidiary, NW Corp, and our operations in South Dakota and Nebraska are conducted through our subsidiary, NWE Public Service. We have provided service in South Dakota and Nebraska since 1923 and in Montana since 2002.

The Gallatin City-County Health Department's mission is to promote and protect health in the communities we serve. We provide essential public health services, prevent disease, and safeguard the environment through proven public health approaches and collaboration. Learn more at healthygallatin.org.

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