Harmful algae blooms can occur in any standing body of water that is exposed to the sun. Some years, under certain conditions, the algal blooms may produce toxins which pose a health threat to animals and people. The algae responsible for algal blooms occur naturally and during spells of warm weather can multiply sufficiently to color the lake water so that it appears green, blue-green or greenish-brown. During calm weather, the algae can rise to the surface to form a scum which may look like blue-green paint, or jelly, and may form flocs or mats on the lake surface.
The Gallatin City-County Health Department recommends that the public avoid swimming in waters containing a green, floating scum of algae. We also recommend that you do not allow domestic animals (livestock and pets) to drink water containing this green algal scum. In Gallatin County, harmful algal blooms have occurred in the past at Hebgen Lake, and the deaths of domestic animals have been attributed to ingestion of water during those harmful algal blooms. Hebgen Lake is routinely monitored to detect the presence of any harmful algal blooms.
For more information, see the Montana Department of Environmental Quality’s Toxic Algae Factsheet.
More information on Healthy Swimming/Recreational water can also be found here: http://www.cdc.gov/healthywater/swimming/index.html