Her mother died in the crash, but the girl pulled through despite frigid water rushing by just below her head and "bleak" weather, Lt. Matt Johnson of the Spanish Fork Police Department told CBC's Carol Off.
The cause of the crash isn't clear, but police believe the toddler's mother, 25-year-old Lynn Jennifer Groesbeck, was involved in some sort of accident around 10:30 p.m. Friday evening. Her vehicle plunged off a bridge into a river, where it landed upside down and was largely obscured from the view of passing vehicles.
Emergency crews made their way to the vehicle almost 14 hours later, just before 12:30, after a fisherman reported it, Johnson said in an interview on CBC Radio's As It Happens.
When the workers first approached the vehicle, they thought it was abandoned, Johnson said. They spotted the driver first, then realized there was a child in the rear passenger side.
"She was non-responsive, but one of the police officers advised that he noticed that she was fluttering her eyes so that's when he realized that she was alive," Johnson said.
The toddler was upside down in the car seat when they found her — but the way the car landed may have helped her survive the night.
"The vehicle was positioned in such a way that the water was only inches from reaching her, but the way the vehicle was positioned on the embankment of the river, it kept the water from actually reaching her."
He said Lily was dressed in fleece pants and a fleece "wind-breaker type" top that kept her "warm enough through the bleak weather for that evening" in which the temperature dipped below the freezing mark.
"Although it’s tragic, there is a bright spot with it — that she was located alive," Johnson said.
Jill Sanderson, Lily's aunt, thanked people for their support and said in a statement posted to a fundraising page that the toddler's improvement has been "astounding" so far.
"Right now she's watching Dora and singing Wheels on the Bus with Grandpa. She is smiling and laughing for family members. We're blown away by Lily's progress and so grateful to her rescuers," Sanderson said.
Police are still waiting for a toxicology report, but Johnson said they don't suspect drug or alcohol use by the mother, and there's no immediate sign of a mechanical failure.
"We don’t know if it’s driver error, if she was distracted or was drowsy," he said. "We may never know why the vehicle left the roadway."