Ms Chamberlain, who has lived in Florida, saw a social media post the turtle and "immediately recognised" the species.
"I suspect somebody has bought it and not realised what it is, it has got too big for them to look after or they cannot afford to feed it," she said.
While it is not illegal to own an alligator snapping turtle, vets do not recommend keeping them as pets due their complicated needs.
Ms Chamberlain went to retrieve the turtle with the help of residents.
"I was anxious on two levels," she said.
"One was actually catching it without losing a finger, but also, what was I going to do with it?"
She wore three pairs of builder's gloves and used a shopping basket to scoop the turtle out, where she transported it to a large container with water from the tarn.
"Everybody rallied round to help", she explained.
Dr Moule, a veterinary surgeon from Wild Side Vets in Barrow-in-Furness, described the discovery as "extremely surprising".
"At this size, it could give you a nasty bite but when they get bigger, they can grow up to 80kg (12st) and they can cause some damage," he said.
He added that the reptile would be "more dangerous for the environment because they will pretty much eat anything".
"I'm sure if it was left there it would be invasive and eat everything in the water," Dr Moule continued.
The turtle is being cared for by the veterinary centre before it is rehomed in a zoo or sent to a private keeper.
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