Two girls were injured when they fell 25 feet down a ravine into a rocky stream in Bolton, Greater Manchester, in the early hours of Saturday.
They jumped over a wall at Dean Brook, Barrow Bridge, not knowing there was a drop on the other side, police believe.
The two, aged 15 and 16, from Bolton, were with two boys at the time.
One of the girls is being treated for spinal injuries at the Royal Bolton Hospital, but the other is not thought to be seriously injured.
Police, fire, ambulance and Bolton Mountain Rescue teams were called to the accident, which happened at just after 0230 BST on Saturday.