State officials are warning people not to swim in the San Luis Reservoir and to avoid eating fish from its water.
The Department of Water Resources on Friday upgraded the advisory for blue-green algae in the reservoir to danger after testing near the Basalt boat ramp earlier this week.
The algae is toxic and exposure can cause skin rashes, eye irritation, mouth ulcers, vomiting, diarrhea or cold and flu-like symptoms.
The state is advising people not to make contact with the water, drink it, or eat mussels or other seafood from it. The same cautions should apply to pets.
The advisory does not apply to the nearby O'Neill Forebay.