Catfishing a river mouth?

I am taking my buddy out cat fishing tomorrow night, we both really want to catch a monster but have failed so far. We are fishing a lake where a river feeds out one end and into lake michigan, we usually throw out big minnows and catch great pike after dark but still haven't caught a nice cat, and i know they are in there because i have seen them when we bow fish for carp. We park the boat in about 15 feet of water and send out a big river chub and sink it to the bottom, any tricks and tips for catching a big cat where a lake flows out and into a river(this seems to be where the big boys have been caught in the past).