Harmful Algal Blooms (HABs) In Montana

Harmful algal blooms (HABs) occur when algae and/or cyanobacteria colonies grow rapidly under favorable environmental conditions. Some HABs produce toxins that can make people, pets, livestock, and wildlife sick. In some cases, these toxins can be fatal.

Cyanobacteria, often called blue-green algae, produce most of the toxins associated with freshwater HABs. Warm water, slow-moving or stagnant flow, abundant sunlight, and excess nutrients can all promote HAB development.

Not every algal bloom is harmful, and not every bloom contains toxin-producing cyanobacteria. Scientists also do not fully understand what causes toxin-producing cyanobacteria to produce or release toxins. Because harmful blooms cannot be identified by appearance alone, people should avoid contact with blooms and follow local health advisories.

To learn more, or if you suspect you have identified a HAB, please click here for additional information. Please also call the Gallatin Local Water Quality District and notify someone on staff!

The MT Department of Environmental Quality has also created a helpful HAB Fact Sheet.

HAB in Hebgen Reservoir, late July 2026. Photo taken by Chris Boyer, @kestrelaerial on Instagram and at lighthawk.org
A large bloom in Lake Helena during late June 2026 consisted primarily of the cyanobacterium Aphanizomenon, a genus that includes toxin-producing species. Other cyanobacterial genera in Montana that can produce toxins include Dolichospermum and Microcystis. Harmful algal blooms often contain a mixture of cyanobacteria and other algae rather than a single species.
HABs often resemble spilled, green paint. Photo of a summer 2020 HAB near the Hyalite Reservoir spillway.
Bright aqua colored spots like this are a tell-tale sign of blue-green algae. When the cyanobacteria colonies dry out and are exposed to sunlight the cells die and burst, releasing a brilliant blue water-soluble protein pigment called phycocyanin. When this bright blue pigment mixes with the remaining green chlorophyll inside the dying cells, it creates a striking turquoise or light blue color on rocks and shores.
Another Hyalite reservoir HAB, this one formed in summer 2021.