Between the peat and the likely rise up the sides of the ledge, not more than four feet deep I think. We're within a mile of the bay so maybe not as cold as it gets inland, but still pretty cold.
My sister had a coi pond in inland Ct, and it wasn't more than eight feet in diameter, and two feet deep, but they made it through for years.
I see your point about leaky peat, but thought that a 50' long run might clog up eventually.
Thanks for the response.