Swimming is already prohibited at the lake, but the Metropolitan Water District has been handing out fliers urging boaters to keep their children and service animals away from the water.
The algae, which has turned the reservoir water a shade of green, has also been found at other Southern California lakes in the past.
Scientifically known as cyanobacteria, the foul-smelling algae thrives when warm, shallow waters are exposed to high external temperatures and light.
Diamond Valley Lake will remain open throughout Father's Day weekend, according to Metropolitan Water District spokesperson Bob Muir.