According to WRCB-TV (http://bit.ly/TaeDKj), sheriff's detectives said the child did not drown.
Someone passing by saw the white vehicle in the river and called police about 8 p.m. Saturday.
"The vehicle had crossed the river, turned around and was coming back when it got stuck in the rocks," said Marion County Sheriff Ronnie Burnett on Monday.
The boy's body was in the vehicle. There was no trauma to the body and no obvious reason he had died, the sheriff said. The boy's body was sent to the state medical examiner's office in Nashville for an autopsy.
"He was 9-year-old white male found in the back seat of the Jeep," Burnett said.
The sheriff confirmed the boy was the son of a woman arrested at the scene, but declined to release his name this early in the investigation.
Police charged the driver, Nicholas Mosier, with DUI, possession of prescription drugs, felony reckless endangerment, and aggravated child abuse and neglect as well as traffic charges. A passenger, Lindsey Newsome, faces aggravated child abuse and neglect, felony reckless endangerment and also a prescription drug charge.
Both Mosier and Newsome live in Whitwell. Both were held in the Marion County Jail Monday and there was no record of attorneys for them.
Detectives said the river water was deeper than usual when the vehicle went in and the high water caused the engine to stall.
The sheriff, the Tennessee Highway Patrol and the Tennessee Bureau of Investigation are looking into the boy's death.
Information from: WRCB-TV, http://www.wrcbtv.com/