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Harmful Algae Blooms And Ways To Report Them

Don’t let an encounter with a harmful algal bloom (HAB) ruin one of the few days of Montana’s short summer. HABs can produce liver and neurotoxins that can sicken humans and even kill pets and livestock if contacted or ingested.

Harmful algal blooms happen when cyanobacteria, also known as blue green algae, rapidly grow out of control. These organisms are native to Montana and are naturally found at low, safe densities in many freshwater systems. HABs can form under certain conditions, such as when too many nutrients are available, when winds are low and water is calm, when the sky is clear and sunny, and when the growing season is long or hot. Toxins do not always occur with a HAB, but it’s impossible to tell without taking water quality samples. When in doubt, it’s best to stay out.

HABs most often occur in Montana’s reservoirs and lakes, although water users should also be cautious about water flowing immediately downstream of a HAB, or if mats of algae are noticed growing on river bottoms. Common characteristics of HABs include green, blue, or gold coloration and the appearance of grass clippings or paint spilled on the water’s surface.

Montanans can submit reports of suspected HABs at:

The website also has a map of all other reports submitted, any health advisories or monitoring data associated with that report, and plenty of photos and information so you can educate yourself on what to look for. If you suspect a HAB, submit a report and the State HAB Team will work with the local jurisdiction to monitor the incident. These reports are important for the health and safety of recreators and water users in the state, and they also help state agencies track where nutrient pollution may be an issue.

Last summer, 42 out of 64 citizen reports were confirmed to be harmful algal blooms. One of those reports was associated with a dog’s death and subsequent closure of a small subdivision pond in Bozeman. Waters that commonly experience HABs include but are not limited to: Clark Canyon Reservoir (Beaverhead County), Cooney Reservoir (Carbon County), Hebgen and Hyalite Reservoirs (Gallatin County), Beaver Creek Reservoir (Hill County), Canyon Ferry, Hauser Reservoir, and Lake Helena (Lewis and Clark County), Harrison Reservoir (Madison County), and Nevada Creek Reservoir (Powell County).

Nutrients, like nitrogen and phosphorus, fuel algae growth. Besides being aware of what HABs look like and the potential health risks, it is also important to understand actions you can take to reduce nutrient pollution in Montana’s waters.

  • Reduce your use of lawn or crop fertilizer.

  • Avoid trampling streamside vegetation when you’re recreating.

  • If you’re lucky enough to live alongside a stream or lake—restore and protect native woody plants, flowers and grasses because they filter pollution and stabilize land.

  • If your home relies on a septic system, have it regularly serviced—even if you don’t live right near a stream or lake. Improperly functioning septic systems can leach bacteria and pollution into groundwater.

When in doubt, stay out. Do not drink, swallow, or swim in water that shows signs of a HAB and keep kids and pets or livestock out. Direct contact, ingestion or inhalation of cyanotoxins may irritate the skin, eyes, nose, throat and respiratory system, or cause nausea, vomiting, diarrhea, headache or liver and kidney damage. If you suspect a HAB-related illness in a person or animal, including livestock, call Poison Control immediately at 1-800-222-1222.

Report a suspected HAB at, call 1-888-849-2938, or by email to

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