The Mazda hatchback was spotted just before 11 a.m. by Caltrans workers on state Highway 29, CHP Officer Marc Johnston said.
The male driver, who was alone in the car, was dead, Johnston said.
It was not immediately clear when the crash occurred, but it was believed to have happened sometime in the past few days, Johnston said.
No other vehicles are believed to have been involved in the crash.
The incident remains under investigation by the CHP.