The victim was with a friend at Steelhead Beach Regional Park around 6:30 p.m. when the incident occurred, Sonoma County sheriff's Lt. Steve Brown said.
Brown said the victim had told his friend that he could not swim but that was going to get in the water at Steelhead Beach to rinse off.
The victim walked downstream in shallow water, and when he didn't return right away, his friend called 911, Brown said.
Sheriff's deputies and the Forestville Fire Protection District responded and found the victim in three feet of water about 50 yards from where he had gone in, Brown said.
An autopsy on the victim is scheduled for today. The man's name is being withheld until his family has been notified of his death.