The girl was in serious condition when ten lifeguards and two scuba divers recovered her body from about 10 to 15 feet of water after searching for nearly an hour, according to a news release.
However, life-saving efforts were unsuccessful at Riverside University Health System Medical Center, where she was later pronounced dead.
Authorities said the girl was last seen swimming in Power Cove, a non-swimming area of Lake Perris meant only for boaters. The incident is under investigation.