The State Water Resources Control Board conducted a field investigation and discovered toxic algal mats growing at the bottom of the Russian River at Riverfront park and in Cloverdale.
The toxic algal mats are detaching from the riverbed and floating on the surface of the water or getting stuck on the riverbanks, the State Water Resources Control Board said in a release.
The State Water Board and Sonoma County health officials warn boaters and recreational water users that these mats "are a threat to human health, especially children, and may be fatal to dogs playing in the water."
Sonoma County has placed signs which read "Toxic Algae Alert" in recreational areas along the river for guidance. The algal mats come in multiple colors ranging from bright to dull-green, orange, brown and maroon.
The algal mats may also cause the wildlife in the water to get infected.
The State Water Board cautions those fishing to throw away all fish guts, clean fillets with tap or bottled water and to avoid shellfish altogether.
It is advised to not drink or cook with the river water and to avoid touching the algal mats.