Authorities said the collision between a personal watercraft and a boat happened sometime around 4:30 p.m. in the launching area near 250 Puddingstone Drive.
Three women, all from the same family, died at the scene. Authorities said the women were riding a three-seat personal watercraft when they were hit by a 20-foot powerboat. Their ages and identities were not immediately available.
The four people injured were on the boat. Three suffered moderate injuries and were taken to Los Angeles County-USC Medical Center for treatment. The fourt person, who had minor injuries, was also taken to the hospital. All of the injured were expected to survive.
Authorities said the crash apparently happened as both were entering a turn.
"The circumstances surrounding the incident are currently under investigation, but it appears both vessels were engaged in a turn, and as they were turning one vessel collided with the other," said Hugo Maldonado, the chief lake lifeguard. "We're taking this very seriously."
It's not yet known if speed was a factor. There was no indication that alcohol was involved.
Officials said it's been years since there was a fatality on the lake.