California water officials are warning residents and visitors to beware of a dangerous plant found in swamps and rivers as summer arrives.

“Beware of the swamp plant. It’s pretty, pretty terrible!” the California Department of Water Resources posted on X June 6, along with a picture of a creepy hand emerging from the mud.

The water department asked the public to watch out for the water hyacinth and other nonnative aquatic invasive species that can be hazardous for boaters and swimmers.

Nonnative species of water plants, quagga mussels, and zebra mussels are invading the state’s Sacramento–San Joaquin Delta and reservoirs and lakes, the water department reported.

Aquatic vegetation damages marina structures and boat launches. The plants become dense mats of weeds and create hazards for boaters, anglers, and swimmers.

Quagga mussels are small freshwater mussels that have invaded North American lakes and streams. They can clog water intake structures, such as pipelines and screams, and interfere with power and water treatment facilities.

Zebra mussels are also invasive mollusks that are fingernail-sized and outcompete native species for food and space. They can overwhelm the water system because of their fast reproduction.

The mussel shells can also leave beaches unsuitable for recreation because of their sharp edges.

“These pests can increase dramatically under the right conditions, displacing native species, clogging waterways and creating hazardous conditions for navigation and recreation,” the water department reported on its website.

Once introduced into a lake or stream, the species are nearly impossible to eradicate. The mussels and plants can also damage boats when they get caught on propellers or attach to hulls.

“Controlling these aquatic invasive species is a multi-million dollar problem in California,” the water department said.