HABs, or Harmful Algal Blooms, are toxic to animals and humans because they produce toxins. Swimming in bodies of water with HABs (or even being NEAR them) can make you sick.

Photosynthetic bacteria are naturally occurring and can be found in many recreational bodies of water.

(The CAUTION on Hyalite Reservoir was lifted on August 10th, 2020. That doesn't mean it couldn't be re-instated, however.)

The potentially TOXIC blue-green algae can potentially be found in ANY standing bodies of water (such as lakes and ponds) as long as they have some sun exposure.

SEE THE MAP OF REPORTED HABs HERE (these are reported blooms and not all have been confirmed as HARMFUL.)

These are public reports of algal blooms that may, or may not, have been confirmed by experts as harmful.

Most reports will be evaluated and advisories (caution or closure) may be issued.

However, It is often impossible to determine if toxins are present at any specific time.

Err on the side of caution and use your best judgement to determine if the water is safe for recreation.


  • Potential toxicity can change very quickly
  • It is impossible to determine if an algae bloom is harmful just by looking.
  • If you observe a suspicious bloom, please feel free to submit a report
  • When in doubt, stay out
  • Caution or Closure signs may be observed and if you see one, please read the details. There will be a different set of rules and details, depending on the sign.
  • If you and/or your pet do go into the water, wash yourself off with clean water immediately after.
  • To report a suspected harmful algal bloom: Call 1-888-849-2938
Algae bloom - MT DEQ



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