IIRC, the record catch for a flathead is almost 5 feet in length and nearly 60 pounds. There have been photographs published from the late 1800s and early 1900s that showed catfish caught on trotlines estimated to be 8-10 feet in length and weighing in at around the century mark.
I'd chalk up all the catfish under the dam stories as embellishment if not for a couple of guys I heard it from. They aren't the kind of guys who stretch a story or tell tall tales.
Granted, deep in a muddy river with almost non-existant visibility, perceptions can be distorted. But, they swore to what they said that they saw. And one guy won't go back down there again. No matter how many times he's been asked.
True.