California authorities issued a danger advisory for San Luis Reservoir in Merced County on Friday due to the toxin levels of blue-green algae in the water.
The Department of Water Resources said people should avoid contact with the water and avoid eating fish or shellfish from the lake.
Boating will be allowed but swimming and other water-contact recreation is not considered safe, the department said. Pets should also be kept away.
Blue-green algae, also known as cyanobacteria, can cause eye irritation, rashes, mouth ulcers, vomiting, diarrhea, and symptoms resembling colds and the flu.
The department said the algae conditions can change rapidly due to wind and wave actions that can move the bloom, which may accumulate to form mats, scum or foam.
The department noted that common water purification techniques such as camping filters, tablets and boiling do not remove the toxins.
The reservoir is about 45 miles (72 kilometers) southeast of San Jose in the hills of the western San Joaquin Valley. It is part of a state recreation area that is popular for fishing, boating, camping and picnicking.