Here's a link to information about the ones in my area streams. I researched after seeing one in the river. It was very BIG (maybe 2 ft long)!

According to the information on the link, it eats mostly crayfish: "A common myth is that hellbenders will 'ruin' good fishing streams by eating all the fish. This couldn't be farther from the truth! In fact, if you see hellbenders in a stream, this is an excellent indication that the water quality is still good"

http://www.hellbenders.org/abouthellbenders.html