The East Bay Regional Park District has closed Lake Del Valle in Livermore for swimming due to a bloom of toxic blue-green algae.
Both people and dogs should stay out of the water, district representatives warned.
District staff detected the algae Monday afternoon near the boat launch and sent water samples to a laboratory in Florida for further testing. Preliminary test results showed a toxicity of more than 20 parts per billion, which exceeds the park district's safety threshold.
At five miles long, Lake Del Valle is the largest lake in the East Bay Regional Park system. It is owned and by the California Department of Water Resources and is part of the state water project.
The park, including the marina and campground, remain open. Fishing and boating are safe for visitors, as long as fish are thoroughly rinsed and the guts removed. Boaters should wash their hands after having contact with the water.
Toxic algae can be fatal for dogs, which is why park district representatives strongly urged dog owners to keep their dogs away from the lake until further notice. For people, toxic algae can cause skin irritation, diarrhea, vomiting, headaches and other symptoms.
Toxic algae continues to plague other lakes in the district, including Quarry Lakes in Fremont and Lake Chabot in Castro Valley.
Toxic blooms have cleared and warning signs have been removed at Lake Temescal in Oakland and Lake Anza in Berkeley, but the lakes remain closed for the season until April 2016.