Police say they received a call from a park ranger at 8:15 p.m. Sunday that CPR was in progress on a man who had been swimming by the docks.
City spokesman Josh Palmer says witnesses reported the man, who was not a proficient swimmer, jumped into a deep part of the lake and was under water for a couple of minutes.
He was pulled out of the water by bystanders. Palmer says the man was unconscious when emergency responders arrived. They used an automatic defibrillator.
The man died at a Twin Falls hospital. His name and age haven't been released.
Palmer says lifeguards won't be on duty in the swimming area until Friday.