Teenager Leah was one of the four people seriously injured on Tuesday when two carriages on the Smiler ride collided. A fifth person is also being treated for internal injuries.
The 17-year-old from Barnsley in South Yorkshire, who was on a first date with fellow victim Joe Pugh, also suffered a fractured hand.
David Washington, Leah's father, said medics "saved Leah's life" following the crash, after which she was said to have been given a blood transfusion and morphine before she passed out.
In a statement to press, her father David Washington said medics "saved Leah's life", after which she was said to have been given a blood transfusion and morphine before she passed out.
"We would like to thank our family and friends and all the well-wishers for their support.
"Leah has suffered a life-changing injury and now has many months of rehabilitation ahead of her.
"We have done this to put people's minds at rest and we would also ask everyone to respect Leah's privacy as she undergoes this rehabilitation.
"We would like to thank all the emergency services at the scene and all the hospital staff who saved Leah's life."
Joe Pugh suffered two broken knees and extensive hand injuries. He recently tweeted out his gratitude for the public's support.
So overwhelmed with the response I've had from my accident at Alton towers, Thankyou for everyone's concern— Joe (@JoePugh_) June 4, 2015
Vicky Balch was forced to spend her 20th birthday in hospital after having both of her legs crushed, and has been described as being in a "serious but stable" condition.
The 20-year-old from Leyland, Lancashire, suffered potentially life-changing injuries in the crash and is expected to make a "substantial claim for damages" to support her recovery, her solicitor said last week.
Daniel Thorpe, a 27-year-old hotel assistant manager from Buxton in Derbyshire, also remains in hospital.
On Monday, Alton Towers reopened to the public after shutting its gates following the crash, which injured 16 people in total.
Nick Varney, chief executive of Merlin Entertainments, said the incident was a "terrible event" for everyone involved.
He added: "We are very aware of the impact it will have on those involved and we are doing all we can to provide our support to those injured and their families.
"We closed the park immediately whilst preliminary investigations took place and to give our staff time to come to terms with the accident and its aftermath.
"In recent days our management team have been engaged in a thorough review of our operating and safety procedures before making this decision."
The park added it was "deeply saddened" by news that rollercoaster crash victim Leah Washington has had her left leg amputated, adding that it will "provide full support to all of those involved now, and throughout their recovery and rehabilitation".
The X-Sector of the Staffordshire theme park - which houses The Smiler - will remain closed until further notice to allow the Health and Safety Executive access to the ride for investigations.
Two rides at other Merlin Entertainments parks, Thorpe Park, and Chessington World of Adventures - both in Surrey - will also remain closed until new safety protocols can be implemented.
Alton Towers was accused of not ringing emergency services soon enough, after it was revealed staff waited 10 minutes before making the first 999 call.
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