A toddler who was rescued after being trapped for more than 14 hours inside a car that had plunged into a Utah river was in stable but critical condition on Sunday, police said.

The 18-month-old girl was flown to hospital after she was discovered by a fisherman on Saturday, Spanish Fork Police Department Lieutenant Matt Johnson said.

Lynn Jennifer Groesbeck, the girl's 25-year-old mother, who was the driver, died in the accident.

Police said the accident appears to have occurred at around 10:30pm. local time on Friday as Groesbeck and her daughter were driving home to Springville, a suburb of Provo. A witness near the site heard a noise and investigated but observed nothing at the time, police said.

The car was discovered by a fisherman shortly after noon on Saturday, with the vehicle resting face-down in the water. When authorities arrived, they discovered the girl in her car seat, police said.

The car appears to have struck a cement barrier on a bridge before entering the river, police said, adding that the cause of the crash was not clear.

Authorities said the name of the girl would not be made public.