People may want to think twice before taking a dip in the green-tinted water near the Parrotts Ferry Bridge at New Melones Reservoir, according to U.S. Bureau of Reclamation officials.

The water’s greenish hue is due to a cyanobacteria bloom that was first detected in the Middle Fork of the Stanislaus River upstream of the reservoir on July 17.

Test results from three water samples taken on July 24 confirmed the presence of cyanobacteria, but did not detect toxins sometimes produced by the organisms that can be harmful to people and animals.

Nevertheless, signs have been posted along the affected area in the river from the bridge to the log-jam at Camp Nine that advise people to avoid the algae and scum if they do go in the water.

“A cautionary advisory is in place to encourage water users to remain out of the affected area until the bloom subsides,” said Cindy Davenport, park manager of New Melones.

Cyanobacteria blooms can be harmful to humans, their pets, and other animals when they produce cyanotoxins, which the CDC says are among the most powerful known natural poisons.

However, there’s no way of knowing if the blooms are producing the toxins just by sight.

Symptoms of cyanobacteria poisoning in people can include irritation of the skin, eyes, nose, and throat, abdominal pain, headache, vomiting, diarrhea, and neurological issues.

Children are most at risk of getting sick. The CDC advises people to wash themselves thoroughly if they come into contact with a harmful bloom, and call a doctor or poison center if they swallow the water.

People are advised to call a veterinarian if their pet drinks or comes into contact with cyanobacteria and exhibits symptoms that include loss of appetite, loss of energy, vomiting, stumbling and falling, foaming at the mouth, diarrhea, convulsions, excessive drooling, tremors, seizures, or any other unexpected sickness.

Davenport said recreation is not affected in other areas of the reservoir, the fourth largest in California with a surface area of roughly 20 square miles.

The bureau is working with the State Water Resources Control Board, Central Valley Regional Water Quality Control Board, and public health departments in Tuolumne and Calaveras counties to get the word out about the bloom.

Cyanobacteria, also called blue-green algae, are microscopic organisms that can be found in all types of water, according to the Centers for Disease Control and Prevention.

Blooms can occur in warm, nutrient-rich environments that allow the cyanobacteria to multiply quickly. They are most common in late summer or early fall and can last for days, weeks, or months depending on the conditions.

The federal Environmental Protection Agency says blooms can also be caused by an unnatural infusion of nutrients in the water from human activity, such as nitrogen and phosphorus from fertilizer, sewage discharge, or stormwater runoff from roads and sidewalks.

Contact Alex MacLean at or (209) 588-4530.