Public Health & Safety: Harmful Algal Blooms
(Recommend Firefox or Chrome)
A bloom of potentially harmful algae appears as 'pea soup,' 'grass clippings,' or 'spilled paint.' The cyanobacteria usually are suspended in the water column or aggregated into floating mats. The surface of the water will appear green, blue, gold, or red.
View the slideshow of photos at the top of this page for examples of cyanobacteria blooms.
suspect a bloom, avoid yourself and others touching or ingestion of the water. Avoid inhalation of aerosolized (particle-sized droplets) of water. If you and/or your pet do go into the water, wash yourself off with clean water immediately after. Report the suspected HAB with a photo at HAB.mt.gov.
Symptoms can start within a few hours of exposure and last for 2-3 days. In humans and animals, symptoms may include: skin, ear, eye, nose, or throat irritation; respiratory issues; lethargy, paralysis, tremors or seizures; abdominal pain, diarrhea, or vomiting. If you are concerned about a potential HAB-associated illness, contact your healthcare provider or call the Montana Poison Control Center at
These temporary advisory signs may be posted by the appropriate authority:
Please email HAB@mt.gov if you are interested in permanent advisory signs.
This map displays harmful algal bloom reports made by the public and any subsequent advisories recommended by the State HAB Program and local jurisdiction. HABs are increasingly common across Montana, and their presence and potential toxicity can change very quickly. Therefore, this map does not show the full potential of waterbodies experiencing blooms, nor can we accurately know the health risk at any given time. Please use your best judgment to determine if a bloom is in the area, and if in doubt, stay out. If you observe a suspicious bloom, please feel free to submit a report to keep the system updated.
For more information on HABs click on one of the following links:
- DEQ's HAB factsheet for the general public
- EPA's Harmful Algal Bloom Webpage
- CDC's Harmful Algal Blooms and Associated Illness
Bring yourself up to speed on HABs by watching these videos:
- See a Bloom, Give it Room: Montana's Capital High School's award winning video (2 min)
- Nutrient Pollution 101 (2 min)
- The Choice Is Yours: Green or Clean? (1 min)
- Protect Your Pooch (15 sec)
- Science Safeguards Drinking Water From Harmful Algal Blooms (3 min)
- Science Safeguards Drinking Water from Harmful Algal Blooms (3 min)
- Algal Blooms Can Harm Your Health (40 sec)
- You Can Prevent Nutrient Pollution (45 sec)
- NYSDEC: HAB Identification: Tips and Tricks (7 min)
To report a suspected harmful algal bloom:
• Click here;
• Call 1-888-849-2938; or
• Email HAB@mt.gov