Flatheads can and do spawn in deeper water. We have found them in holes at the end on boat ramps at near 30 feet deep when Texoma was high. I believe they try to return to traditional or previous holes they have used before. I wouldn't expect to find any flatheads in knee deep water right now. Different story with them mean as hell little channel cats that like to bite you all over. You might well find these evil things in the rocks at 3 to 4 ft deep around mid June.
From Genesis: "And God promised men that good and obedient wives would be found in all corners of the earth."
Then He made the earth round...and He laughed and laughed and laughed!