If you did wade up into that fast section the fish would all spook and disappear. If you stood still long enough they'd all appear again. At that point they'd eat anything you threw at them. "Selective" wasn't happening.
I have a carp pool on the Denver South Platte, where they are both wader and fly shy.. in four years of trying I've caught one (1) carp there. That was on a blind cast at a hazard..
It's possible to wade out, wait 15min without moving and they will reappear, but I've not yet found any fly they will eat..
once on a Wyoming lake, released a brook trout, it swam behind my wader legs and started picking scuds off my waders.. clearly not much traumatized by being caught.
sometimes a #20 RS2
tied with CDC will work for me on the hypersensitive Cheeseman/South Platte trout. This is unweighted, trailed behind some beadhead and/or a splitshot. But usually I just get skunked ;-)