That's an American eel, the only ones we have in TN supposedly, and yes, mostly in the Cumberland River system.
Cool catch, I don't think that common to get on a rod & reel. I've heard you can skin them like a catfish and they're quite good fried. Never had one to try.
But I did pull this off a website, "Take note of the location (and take a picture if you can) and email Tennessee Wildlife Resources Agency biologist Pandy English at
Pandy.English@tn.gov."
Guess he's doing some study on them or something.