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Idaho Fish and Game


Blue-Green Algae Discovered at Horsethief Reservoir


Warmer water temperatures resulting from southwest Idaho’s prolonged summer heat wave are likely to blame for a blue-green algae outbreak at Horsethief Reservoir near Cascade. While levels of the toxin-producing bacteria remain unknown, water samples taken from the reservoir on Tuesday confirmed the presence of blue-green algae.

Horsethief visitors are advised to take appropriate precautions.

Based on current samples, boating and fishing remain appropriate activities according to authorities at the Idaho Department of Environmental Quality. However, Horsethief visitors should refrain from drinking, swimming or otherwise entering the reservoir until further notice. Pets should also be kept away from the water as the algae can cause sickness and even death. Dogs are particularly susceptible to blue-green algae poisoning.

Though water samples tested positive, bacteria levels were determined to be relatively low. Additional sampling will continue as long as the warm weather persists. Watch for additional information should conditions change.

Learn more about blue-green algae and its effects on humans and pets at the following sites:

Department of Environmental Quality -

Southwest District Health -