Riffles are where bugs come from. The choppy, broken surface oxygenates the water, and the rocky bottom is full of mayfly nymphs, caddis larvae, and stoneflies. Trout sit at the head and tail of riffles waiting for that food to wash through.
When in doubt, fish the seam where a riffle dumps into the next pool — the head of the run. That's the cafeteria line. A drag-free dead drift through the foam at the seam catches more fish than anywhere else on the river.