|I often used to look at big rivers and just give up on the areas with big heavy currents. Then a few years ago I watched a guy wade into an area with some serious lean-upstream current on the Green River and start catching fish after fish. Obviously pockets behind rocks, areas with debris, and shorelines that create slower water will hold the majority of fish, but there are fish right out in the heavy water and very often they are big and they are feeding.
Many sections of river that don't appear to be great fishing areas actually have hidden structure on the river bottom that provides current breaks. Large rocks and weeds, even if they don't protrude out of the water or even really form much surface disturbance, can still create fish holding areas. Freestone rivers can actually hold fish almost throughout the river despite the surface currents appearing too swift.
The best way to fish these types of areas is to use very heavy nymphs or lots of weight or even both. Fish in very heavy water probably won't come to the surface. As you fish your nymph rig, you will have to remember that your flies will only be in the fish holding zone along the bottom for a short part of their drift, usually at the very end of the drift, so it also pays to cover the water methodically by fishing several casts in each reachable current lane and then moving one careful step upstream or down(down is easier) and fishing several more casts in each current lane. Good drifts are much harder to attain in heavy water so keep trying different leader lengths and amounts of weight. Also remember that the surface current is moving much faster than the current on the river bottom so avoid drag like the plague with several mends per drift. I usually mend both the line and indicator slightly upstream to avoid the indicator causing drag too.
Strikes in this kind of water are usually quick and very noticeable so you will know when you get one. Also the battles in this kind of current are often epic even from smallish fish. You'll think they should be filming you for "A River Runs Through It 2" on almost every fish.