On a hot August day, the surface of Hebgen Lake might look inviting, but what’s lurking just beneath could pose a health risk.
Not uncommon for late summer, Greater Yellowstone is currently facing a surge in harmful algae blooms. Caused by an overgrowth of bacteria called cyanobacteria that has the potential to produce toxins that can sicken people, pets and wildlife, these blooms have been confirmed in at least four lakes in Montana so far this month.
Neighboring states are also seeing similar patterns. According to the Wyoming and Idaho DEQ, multiple reservoirs and lakes in each state have also been under health advisories due to HABs this summer.
The Montana Department of Environmental Quality has confirmed algae blooms have appeared in Hebgen Lake’s Grayling Arm. as well as Van Houten, Ennis and Homestake lakes.
The Gallatin City-County Health Department issued an advisory on August 21, related to harmful algae blooms, or HABs, detected in Hebgen Reservoir in the Rainbow Point Campground area.
An August 21 press release issued by the Gallatin City-County Health Department, NorthWestern Energy and Montana DEQ urged caution: these blooms can affect water quality and public health, even when they appear in small, localized areas.
Montana DEQ cautions against swimming, drinking or letting pets enter water if it looks like pea soup, has a thick scum, or appears to be streaked with paint. HABs often collect in shallow areas like coves or along shorelines where water is warmer, even when the rest of the lake appears clear.
This summer, Montana’s HAB activity appears to be about normal, says Tiffany Lyden, education and outreach specialist at DEQ. “We average about 35 to 45 confirmed HAB reports each season,” Lyden told Mountain Journal in an email. “Some years are more, some less. The one difference compared to last year is that we’ve received more potential reports that turned out to be something other than a HAB (i.e., green algae, aquatic plants, pollen, etc.).”
Lyden said HABs are usually found in stagnant or slow-moving water, and are less commonly found in rivers, which are typically associated with green algae. According to Montana DEQ, although green algae resembles HABs, it does not produce toxins and is therefore not harmful to human health.
Lyden also noted that because Montana’s HAB data relies heavily on citizen reports, year-to-year comparisons are tricky to pin down. “We can’t really assess trends over time, as increased HAB reports can be due to multiple factors including increased public awareness,” she said.
Harmful algae blooms are rapid overgrowths of cyanobacteria — microscopic organisms sometimes called blue-green algae. Not all cyanobacteria are dangerous, but certain species can produce toxins that affect humans and animals. Exposure most commonly occurs by drinking or swimming in contaminated water.
Symptoms of HAB-related illness can start within hours of contamination and last for several days. In humans, they can include rashes, sore throat, coughing or gastrointestinal issues. In pets and livestock, the risks can be more severe, including vomiting, seizures or death.
These blooms thrive in still, warm, nutrient-rich water — conditions increasingly common across the West. High temperatures, long days of sun exposure and minimal wind help cyanobacteria grow unchecked. Fertilizer runoff and other nutrient pollution can accelerate the process.
Wind or rain can help dissipate a bloom, but the danger doesn’t disappear overnight. Toxins may remain in water bodies for up to two weeks after a bloom visually clears, which means even seemingly clean water can pose a risk.
For up-to-date information on reported blooms, Montana DEQ provides a live map to track the outbreaks. The agency encourages reporting suspected blooms through its website.
If you suspect exposure in yourself or a pet, rinse off immediately and contact a healthcare provider or veterinarian. Montana DEQ recommends avoiding affected areas until at least 14 days after the bloom has fully disappeared.
