Even in the south stretches of the Duper, there are few very spots where a giant like this could seek shelter. This is also the first report I have ever heard of any Flatheads in the Duper, and its obviously been around a while! Buuuut then again... I never would have believed a Musky to be caught in the Duper, if I didnt see it with my own eyes. (last spring)
My guess, is somewhere around Channahon where the Kanankee, Des Plaines, and Dupage all meet. For as low as that water looks, it seems pretty stained for the Dupage.
Regardless of where it was caught, that is a heckuva fish!