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Toxic Algal Bloom Closes Section Of Hebgen Reservoir

Swirls of algae form a sheen on water
NorthWestern Energy
The toxic algal bloom at Corey Spring area at Hebgen Reservoir taken in Aug. 2020.

A section of the Hebgen Reservoir near West Yellowstone is closed due to a toxic algae bloom.

The Rainbow Point shoreline is closed after lab tests Aug. 29 confirmed the presence of a neurotoxin, which is produced after certain types of blue green algae rapidly grow and decay.

People, pets and livestock that swallow or have prolonged exposure to this type of toxin may experience convulsions, paralysis or death.

A spokesperson with NorthWestern Energy, which operates Hebgen Dam and regularly monitors the reservoir for algae blooms, told YPR its hydrology team first detected toxins Aug. 19 in the Corey Spring area, northeast of Rainbow Point.

Rainbow Point area of the Hebgen Reservoir taken in August showing algae bloom
NorthWestern Energy
This image from the Rainbow Point area of the Hebgen Reservoir taken in August shows the algae bloom.

While warning signs have been posted, the spokesperson said the toxin levels in the Corey Spring area do not warrant a closure at this time. The NorthWestern hydrology team will continue to collect water samples for lab analysis.

Algae blooms occur almost every year at Hebgen, usually mid to late August. The last toxic bloom on the reservoir was October 2016.

Montana has had 28 confirmed toxic algae blooms so far this year, according to the state’s health department.

The state’s website has resources to help people identify and submit reports of suspicious blooms, learn about nutrient pollution prevention, and tips on keeping people and animals safe.

The Wyoming Department of Health Aug. 31 also issued a recreational use advisory for Weston Reservoir southwest of Sheridan due to a harmful cyanobacterial bloom. The health department says people should avoid contact with the water in areas where scum is visible.