People are being warned to stay out of the water at Lake Elsinore and Big Bear Lake due to the presence of harmful algae that could be toxic.
The blooms pose a potential health threat to people and pets, according to the State Water Resources Control Board.
Lake Elsinore is open for all recreational uses, including boating, fishing and kayaking - but swimming is not recommended. At Big Bear Lake, there are signs for people and pets to stay out of the water, with additional warnings not to let dogs drink from the lake or eat scum washing up on shore.
Fish caught in the lake should be properly gutted and the fillets cleaned with tap or bottled water, officials said.
"Cyanobacteria, a group of organisms that form harmful algal blooms, can produce potent toxins," according to a Water Resources Control Board statement. "Health risks include skin inflammation, gastrointestinal distress, headaches, agitation and weakness or abnormal breathing if ... material is swallowed while swimming."
Algal growth is attributed to higher volumes of nutrients in the water, combined with elevated temperatures, lots of sunshine and little to no wind.
"Bloom conditions can change rapidly, as the winds and waves move or concentrate the bloom into different regions of the lake," the WRCB stated.
"In some areas, the bloom may concentrate and form a film or scum on the water surface. The color of the water may also appear discolored as bright or dark green and brown."
Officials have not said how long the warnings will be posted.
City News Service contributed to this report.