The 14 year-old was rescued by mountain rescue team volunteers at Badgers Wood after he fell at around 5.30pm on Tuesday, June 26.
A spokesman for the ambulance service said: "A teenage boy in his mid teens suffered arm and leg injuries following a fall.
"He was taken to Alder Hey children's hospital by air ambulance.
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"He was described as having major trauma, although he was conscious when paramedics attended."
A Bowland Mountain Rescue Team spokesman said: "This evening the team were called to assist North West Ambulance Service and North West Air Ambulance Charity with a boy who had fallen from a tree and landed in the river.
"He suffered from lower leg and arm injuries. The team carried him to the air ambulance for a wards transportation to hospital.
"We wish him a speedy recovery."
The current condition of the teenager is not known.