Sonoma County officials are warning people to take care around a popular northern California river after tests found trace amounts of an algae toxin that killed a dog there in 2015.

After tests were done on the Russian River near Sacramento, results showed the blue-green algae toxin known as anatoxin at four public beaches along the river.

County officials have posted warnings to keep dogs and children away.

The toxin can cause muscle tremors and can lead to seizures and death in severe cases.