The incident happened shortly before 3 p.m. at Piedra and Elwood roads. Authorities said the man went underwater while in the river and did not resurface.
Two bystanders found him and pulled him to shore, where deputies and CAL FIRE firefighters administered CPR.
He was transported by ambulance to a hospital, where he was later pronounced dead.
Sheriff's spokesperson Tony Botti said the man was not wearing a life jacket and had consumed alcohol prior to entering the water.
His identity has not yet been released, pending notification of family.