According to the health department, algae in the water around the Charleston Day-Use Area is producing high levels of dangerous toxins.
State health officials said drinking the water could hurt or even kill humans and animals, and any exposure to the water could cause illness.
Visitors are advised not to swim or water ski, and to take precautions if boating or fishing in the area.
Harmful algae are affecting other parts of the reservoir as well. The Wallsburg Area is under a warning.
Health officials say visitors should stay out of the water and clean fish well.
Learn more about harmful algal blooms from Utah's Department of Environmental Quality.
