Nothing ruins a trip to the lake like toxic, life-threatening algae. It sounds like a cynical joke but one of California’s most popular lakes has skiers, fishermen, swimmers and outdoor enthusiasts seeing green.

In 2016, officials from the Central Valley Regional Water Quality Control Board found blue-green algae in the Pit River Arm of Shasta Lake in Northern California. Local news showed footage of how the usual blue water has turned into a murky green that is caused by its reproduction through photosynthesis.

Local, county and state officials are warning lake goers to stay away.

This toxic algae comes from cyanobacteria. This is the oldest bacteria on the planet dating 3.5 billions years. According to the University of California at Berkeley, cyanobacteria is: “important in shaping the course of evolution and ecological change throughout earth’s history. The oxygen atmosphere that we depend on was generated by numerous cyanobacteria during the Archaean and Proterozoic Eras.”

California isn’t the only state affected. Also this month, a toxic algae bloom struck Utah Lake and extended north into the Jordan River system just south of Salt Lake City.  County officials warned residents that the county canals are potentially unsafe for people and animals.

Some of the hazards that come with exposure to the algae are life threatening. Cyanobacteria release toxins that can affect the brain, nervous system and organ functions of animals and humans.

The challenge with preventing this type of toxic algae from growing in waterways is that some of the reasons for the bacterial growth can’t be controlled, like warm weather or stagnant water. According to an official with the Department of Environmental Quality in Utah, nutrient concentration is the only variable that can be controlled.

In time—and with some wind to stir the stagnant waters—the algae should die off on its own. But as the ubiquitous lake vacations of summer roll along, make sure you check with environmental authorities about any potential hazardous toxins that may be lurking at your favorite water destination.  In addition, if your residential water is processed from lake water, be aware that the water processing plants may not filter out all of the algal toxins.