Just my opinion but if Flatheads were going to expand to the upper Delaware, especially the East and West branches, I think we'd already seen it happen to some degree. Just checked the water gauges for the West Branch which is running at 55° today. Water temperature in Phillipsburg is 78°. That is precisely why you won't see flatheads migrate much further up the Delaware than the Water Gap area. Water temps and habitat don't accommodate the environment they're looking for.

East and West branches of the Delaware are extremely cold so again flat heads are not going to be up there as those temperature ranges are not conducive to flatheads. I just don't see it happening in our lifetime but if it does it's going to be due to extreme warming of river temperatures which will pose more of a threat to the fabled trout fishery than flatheads ever will. You'd essentially have to see a complete change in the ecosystem of the river before flatheads venture that far north. Same reason that you don't see trout populations in the Philipsburg area.

Another fact for what it's worth is flat heads grow on average 3" a year so a 30" flat head is about a 10-year-old fish. I would bet the ranch that for the first three to four years of their life cycle there's more flat heads that fall victim to predatory fish than indigenous species that fall prey to larger flatheads. 5 or 6 years ago when a guy trolling in a row boat out of Dick Dow's at Lake Hopatcong caught a 36 lb muskie on a small Phoebe, when F&G examined that fish it had six 6" bullheads in its stomach. I would not be surprised that smaller flatheads are forage fish for walleye, smallmouths, muskies, channels, stripers and possibly even carp more than other indigenous species are forage fish for larger flatheads. My point is until they reach a certain size, they're not the apex predator in the system people think they are but more a vital addition to the forage base that other predators feast on.