I grew up fishing for, catching, cleaning and eating bullheads (and crappies).
I did some checking with the DNR and others who are well informed on fishes.
Bullheads, probably same for any river anywhere that is in bullhead country, are in all the rivers.
Up here bullheads and Northern Pikes are usually the last survivors in many shallow lakes suffering winter kill, and minnows.
I did find out that large bullheads those over fifteen inches plus act the same as large Northerns and Walleyes, they want large deeper water.
Most people never catch them because they do not fish where they are.
In the lake fish surveys MN does every year, though not same lakes every year, some lakes, most in middle Mn where the fishing hotspots are, they have netted , from all three northern species, bullheads in the 20-24 inch category.
One lake produced a bullhead over 24 inches long as it was in the next category.
Often times the same lakes that produced Northerns of over 40 inches and Walleyes of 30 inches or more were the same cold deep lakes hat had a population of truly large bullheads fifteen inches or larger.
Some of these lakes are the deep ones, 80 plus feet that have little shallow water.