DEQ Press Releases

Driscoll, Paul

DEQ Issues Advisory on Blue-Green Algae Blooms

Ponds, lakes, and reservoirs most often affected

HELENA -- The Montana Department of Environmental Quality advises people to avoid swimming in ponds, lakes, or reservoirs that seem stagnated and harbor large quantities of algae. Mid-summer hot weather can trigger outbreaks, or blooms, of potentially toxic blue-green algae.


Blue-green algae, or cyanobacteria, can reproduce rapidly in standing fresh water when sunlight, temperature, and nutrient levels are high. Within a few days a clear lake, pond, or ditch can become cloudy with algal growth. Potentially toxic blue-green algae can occur throughout Montana in lakes, reservoirs, stock ponds, or roadside ditches. These types of blooms are not normally found in moving waters, such as streams, irrigation canals, and springs. Because they require sunlight, the blooms do not occur in wells.


A bloom may appear similar to bright green latex paint floating on the water. However, it can also appear bluish, brownish, or reddish-green, or even look like grass clippings in the water. A bloom is comprised of extremely small organisms usually suspended in water or forming floating mats that can be several inches thick near the shoreline.


In most cases in Montana the algae is not toxic. Only certain species are capable of producing toxins, and even these remain harmless most of the time. Nonetheless, certain blue-green algae can, and have, sickened or killed pets, waterfowl, and other animals, including livestock. They can also cause serious illness in humans.


Signs of a toxic bloom may include the presence of dead fish, waterfowl or other animals, or a sudden unexplained illness or death of a pet, especially if it has algae on its mouth, legs or feet. The definitive test for water suspected of harboring toxic algae is to inject samples into a laboratory animal. Harmless strains of algae and active toxic strains look very similar under a microscope.


A cooperative study is underway in Canyon Ferry Reservoir this field season between DEQ, the Environmental Protection Agency, and the U.S. Geological Survey (Wyoming-Montana) Water Science Center to ascertain whether the blooms can be controlled in the reservoir. Preliminary results of the study may be available in 2016.


If you suspect a bloom in standing waters, keep children, pets, and livestock away. The DEQ has posted a fact sheet about blue-green algae on its website at:

Previous Article DEQ and Flathead County School District Agree to Resolve Underground Storage Tank Act Violations
Next Article Governor Bullock to Montana’s Congressional Delegation: Fight to Preserve Funding for Abandoned Mine Lands