To find out if your river system has a decent flathead population, hit the bait shops. Search out the pics and pick the brain of the proprietor and browse the shelves with your ears open.
Looking for flatheads is like looking for bass, muskie, walleye, in other words predator fish.
The biggest fish, just like people get the nicest digs. Heaven for a flathead consists of food, shelter, food, a natural highway to security and food.
Swimming in current takes energy. To have energy takes food. If food can be eaten without a large expenditure of energy, then the predator can rest more often.
In a discharge (deadly when the surrounding water is less than 50 degrees) you need to see the whole picture. Big flatheads love wood. If there is a log on a flat within 5-10 yards of the current, or a hump, boulder or log blocking current a big flattie will call this home.
While inactive (not biting) the predator will be behind the obstruction resting out of the current. An attentive fish, getting more active will move off to the side and sniff the current (like a land predator sniffing the wind). A hungry predator will move to the front of the obstruction. As the current rushes to an obstruction it will push, create a roll and go over the obstruction making a perfect spot for a predator to watch sniff pounce and inhale a helpless bait stuggling in heavy current.
You keep sticking to one spot. Study your quarry. Listen very carefully. If you want to be a successful catter go shallow. Where do baitfish go when it gets dark? Is it safer to be a gill in shallow water where you only have three sides to watch your back and a shoreline as protection or in deep water where a predator can come from 4 sides, top and bottom to eat you?
Now, do predators know where their food is?
Follow the food and present your bait. I would be willing to bet a large sum of money that many many more big flatheads are caught in less than 5 feet of water than more.
Good luck and welcome to the obsession!