A harmful toxin linked to blue-green algae has been confirmed in Hebgen Reservoir in southwestern Montana, prompting a public health advisory and additional warning signage at the site. NorthWestern Energy announced the detection of anatoxin-A in the Corey Springs area of the reservoir’s Grayling Arm, raising concerns for swimmers, families, and pet owners heading to the water during the height of summer.
What Was Found and Where
Testing revealed anatoxin-A at a concentration of 2.12 micrograms per liter — a level that triggers what officials classify as a caution and warning stage, activating public outreach efforts and the placement of additional signs around the affected area. The toxin is associated with harmful algal blooms, which form when blue-green algae proliferates in warm, shallow water.
Authorities have not yet moved to close any beaches. That step would be considered if toxin levels climb to 8 micrograms per liter or higher — nearly four times the concentration currently recorded.
How Common Is This at Hebgen?
While the advisory may alarm visitors unfamiliar with the reservoir, water quality specialists say the phenomenon is a recurring feature of the site. Jordan Tollefson, a water quality specialist with NorthWestern Energy, noted that this marks the first confirmed detection at Hebgen this calendar year but placed it in longer-term context.
“This is the first time this year. So this is something that doesn’t quite happen annually at Hebgen Reservoir, but I would say maybe three years out of five,” Tollefson said.
Blue-green algae occurs naturally in Montana’s lakes and reservoirs, and conditions at Hebgen — including shallow-water areas where algae tends to accumulate — make periodic blooms predictable rather than exceptional.
Risks to Children and Pets
Health officials and water quality managers are particularly concerned about ingestion risk in shallow areas, where algae can concentrate near the shoreline. Children playing in the shallows and dogs that drink from or wade through affected water face a higher exposure risk than adults swimming in deeper areas. No medical incidents related to the current bloom had been reported as of the advisory’s release.
Families planning visits to Hebgen are urged to watch for discolored water, surface scum, or an unusual odor — common signs that algae concentrations may be elevated — and to keep children and pets out of affected zones.
Who Is Monitoring the Situation
NorthWestern Energy is coordinating the monitoring effort alongside the Montana Department of Environmental Quality, Healthy Gallatin, and the U.S. Forest Service. The partnership reflects a multi-agency approach to reservoir health that has been in place at Hebgen and provides a structured protocol for escalating or de-escalating advisories based on measured toxin levels.
NorthWestern Energy confirmed that water quality testing at the reservoir will continue, with results informing any decisions about expanded warnings or potential beach closures.
What Visitors Should Know
Hebgen Reservoir is a popular recreational destination in Gallatin County, drawing anglers, boaters, and campers throughout the summer season. The reservoir’s proximity to Yellowstone National Park makes it a common stop for tourists, and the advisory arrives during one of the busiest periods of the year for outdoor recreation in the region. Montana’s outdoor tourism economy generated $5.6 billion in visitor spending in the most recent tracking year, underscoring the economic stakes when public health warnings affect high-traffic recreation sites.
Visitors should look for posted signage in the Corey Springs area and Grayling Arm, and consult updates from NorthWestern Energy or the Montana Department of Environmental Quality before entering the water. The current advisory does not constitute a full closure, but officials are asking the public to exercise caution until monitoring confirms that toxin levels have dropped back to safe thresholds.
