Blue Mesa Water Quality Advisory – Toxic Algae Found August 2025

GUNNISON, Colo. - Throughout the summer, National Park Service staff sample and analyze the Blue Mesa Reservoir water for the presence of blue-green algae containing cyanotoxins. The latest testing revealed that concentrations of the harmful algae within the Iola Basin exceed public health safe exposure levels.

Toxins were found at North Willow East, on the east side of Sometimes Island, and that general location has been posted with warning signs. Due to shifting winds and currents, other areas in the Iola Basin of Blue Mesa Reservoir may also contain these toxins, and notification signs have been distributed to heavily used visitor areas in the basin. The park staff advises to avoid areas with suspected algal mats in this area, and throughout the reservoir.

Use caution and avoid unnecessary exposure to reservoir water if fishing, boating, or recreating. Until further notice, the park recommends that no one swims in, or drinks from, reservoir waters in the Iola Basin. Also, do not allow your pets to drink the water in that area. Contact medical care (doctor or veterinarian) if, after exposure to the water, individuals or pets exhibit nausea, vomiting, digestive distress, breathing problems, seizure, or unexplained illness.

Harmful algae, a type of blue-green algae, are common and natural to our waters and are found throughout Colorado. Algae can multiply rapidly to form blooms and scums, particularly in areas of shallow, warm water. Certain types of algal blooms can produce toxins called cyanotoxins, which can be harmful to humans and animals.

Boating and fishing remain open throughout Blue Mesa. Be sure to clean harvested fish with treated (potable) water. As always, remember to clean, drain, and dry all boats and fishing gear.

Water quality monitoring staff continue to sample critical areas and are monitoring conditions. Check the park website for updates at www.nps.gov/cure.

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