DEQ Press Releases

Driscoll, Paul

State agencies continue to conduct outreach and study Harmful Algal Blooms

HELENA – As Montanans head to the lake for summer fun, citizens are urged 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. But scientists believe the blooms are occurring at more locations, with increased frequency and longer duration. Potential 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 and animals sick, and at worst, exposure can be fatal.


Importantly, “children and pets are more likely to ingest these HAB infested waters as they spend most of their time wading in the shallows where the algae can accumulate, and they have less control over how much water is ingested,” said Matt Ferguson, State Toxicologist at DPHHS. Not all varieties of blue-green algae are harmful, but some, under the right conditions, 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 column or aggregated into floating mats; they do not grow from the bottom as do roots, mosses or water plants.


“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 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 help by reporting HABs so that we can respond quickly and hopefully prevent people, pets, and livestock from getting sick.” 


Citizens can submit reports of suspected HABs, including photos, to the website: The site also has a live map of reported HABs and how-to identification information. When a HAB is reported, DEQ investigates to determine whether the bloom is nuisance green algae or potentially-toxic blue-green algae. DEQ then works with the entities managing the waterbody to determine whether further water quality testing is needed and if safety advisories are warranted.


During the 2018 growing season, the Montana HAB Program received 25 percent more reports of HABs on Montana’s lakes, reservoirs and ponds than the previous year. Fifty of the 60 reports received were confirmed to be blue-green algae, although toxicity tests showed cyanotoxins to be at safe levels. Two thirds of the reports were in the Helena Valley area, including Canyon Ferry Reservoir, Holter and Hauser Lakes, and Lake Helena. Elsewhere, HABs were reported on Delmoe Lake and the Clark Canyon, Nevada Creek, Yellow Water, Cooney, Nilan, Hebgen, Hyalite and Beaver Creek (near Havre) reservoirs.


HABs are receiving increased attention nationally because of the potential human health and economic impacts. In 2018, drinking water advisories were issued for several weeks for approximately 150,000 residents of Oregon communities near the Detroit Reservoir. In Montana, reports of livestock and pet illness and death have been reported since the 1970s and as recently as 2018.


In addition to supporting the Montana HAB Program, DEQ is developing numeric nutrient standards for lakes, including Canyon Ferry Reservoir, which will help guide management decisions that may lessen the prevalence of HABs.


Suspect a HAB?

When in doubt, stay out. 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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