The lifeguard told police he was going only 5 mph and did not see the girl because she was under a towel at Pacific Beach.
The 3-year-old's father had left her there. The experienced lifeguard was shaken up by what happened.
"It is a nightmare situation," said Chief Lifeguard Rick Wurts. "No one wants to be dealing with something like this."
"They had the neck brace on her and everything," said witness Casey Kwasek. "She was moving so we assumed she was OK."
The toddler was taken to a local hospital with minor injuries. Police are investigating the incident. They confirm the lifeguard was driving slowly and say the girl's father was surfing at the time of the accident.