DEQ Press Releases

Davin, Moira

Warm Weather Increases Risk of Harmful Algal Blooms in Montana Lakes and Reservoirs

HELENA—Summer is in full bloom. Before you take a dip in the lake this summer, state agencies are urging Montanans to know the health risks of Harmful Algal Blooms, or “HABs.” 

HABs are caused by blue-green algae that are native to Montana’s freshwater lakes and reservoirs. Scientists believe the blooms are occurring at more locations, with increased frequency and longer duration. Causes include warmer water temperatures, longer summer growing seasons and increased nitrogen and phosphorus runoff from a variety of human activities. Under certain conditions, HABs can produce toxins that damage the skin, liver and nerve cells. They can make people sick and kill pets and livestock. 

Not all varieties of blue-green algae are harmful, but some can produce dangerous cyanotoxins. Blue-green algae blooms often look like pea soup, grass clippings or green latex paint. The algae are usually suspended in the water or aggregated into floating mats. Blue-green algae are single-celled organisms, so they will not appear thread-like or stringy like roots, mosses, water plants, or green algae. 

Since 2017, Montana’s HAB program partners–the Department of Environmental Quality and the Department of Public Health & Human Services–have enlisted the public’s help to track suspected HABs.

“The goal of the Montana HAB Program is to educate people about the potential dangers of HABs, how to identify them and how to respond,” said Hannah Riedl, water quality specialist at DEQ. “Because we can’t be there to track rapidly changing conditions in every water body, we want people to be informed enough to make their own decisions. We also ask the public to be our eyes on the ground and report HABs. That way, we can respond quickly and hopefully prevent people, pets, and livestock from getting sick.”  

How can I tell if there is a HAB? 

Cyanobacteria can look like spilled paint, grass clippings, or pea soup, and can be green, blue, red, brown or gold. A water sample is required to determined toxin presence. Harmless green algae also occurs in freshwater, but does not produce harmful toxins. Green algae is filamentous, stringy or moss-like.

Follow DEQ social media channels (@MTDEQ on Facebook and Twitter and @MontanaDEQ on Instagram) for more photos of HABs and information.

Help keep Montanans safe and report suspected HABs, including photos, to

The website has identification information and an up-to-date map of reported HABs so anyone can report and see where HABs are present. When a HAB is reported, DEQ investigates to determine whether the bloom is a potentially-toxic blue-green algae or something else, like a nuisance green algae bloom. DEQ then works with the entities managing the waterbody to determine whether further water quality testing is needed and if safety advisories are warranted. 

Where have HABs been present in the past?

Between June and October of 2019, the Montana HAB Program received 48 reports of HABs on Montana’s lakes, reservoirs and ponds. Last year, two temporary closures were issued due to HABs. The Cow Creek Reservoir (Blaine County) temporarily closed due to microcystin levels of 50 mg/L, well above the recreational guidelines of 8 mg/L. Additionally, White Sandy Beach on Hauser Lake (Lewis and Clark County) temporarily closed due to microcystin levels detected with test strips. Other lakes and reservoirs that were visually confirmed in 2019 to have cyanobacteria present were Cooney Reservoir (Carbon County), Wadsworth Pond (Cascade County), Hebgen and Hyalite Reservoirs (Gallatin County), Bearpaw Lake and Beaver Creek Reservoir (Hill County), Canyon Ferry and Holter Reservoirs (Lewis and Clark County), Seeley Lake (Missoula County), Noxon Reservoir (Sanders County), Medicine Lake (Sheridan County) and Lake Elmo (Yellowstone County).

What are the health impacts of HABs?

HABs are receiving increased attention nationally because of the potential human health and economic impacts. Reports of livestock and pet illness and death have been reported since the 1970s, including as recently as this year.

“During a pandemic it’s especially important to take precautions to protect the health of yourself and your loved ones,” said Ed Evanson, supervisor of the DPHHS Food and Consumer Safety Section. “When in doubt, stay out. Keep a close eye on pets and livestock to ensure they don’t drink the water where HABs are present.”

Do not drink, swallow, or swim in water that shows signs of a HAB, and be sure to 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.

Learn how to report a suspected HAB at or call 1-888-849-2938. You may also report a suspected HAB by email to


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