It plunged down a steep bank and flipped upside-down into the fast-flowing freezing currents of the River Clough.
The terrified mum and daughter were trapped as water flooded in.
Diane was upside-down, unable to free her seatbelt, and with her head and shoulders submerged in the icy water.
But Sarah, just 14, managed to flip herself around, then drag her mother from under the water and kick open the jammed driver's door.
Exhausted, she pulled her mother out and they waded to the bank and scrambled up it, but their ordeal wasn't over.
Diane passed out and Sarah had to revive her before trekking across fields for help.
Sarah, from Hawes, North Yorks, says that remembering a similar incident on the 999 Lifesavers TV show helped her escape.
She has been nominated by her aunt, Christine Bell, who says: "The police said that if it hadn't been for Sarah's courage and clear head, my sister would not be here today.