FYI. Central Oregon and Sunriver Fly clubs do annual classes (Kokanee Karnival) for grade schoolers on the Metolius. To show what fish feed on we'd disturb the bottom and catch the downdrift in a screen. Usual count of insects from a 2'X2' area on the bottom included: 75+ shelled caddis, 50+ stonefly nymphs, 10-15 shellless caddis, 5-10 San Juan worms, 150+ various others. This was done in the gravel/rocky bottom area above Camp Sherman at the walk-in campground. Just about every afternoon, about 2, there is a March Brown hatch about size 14. A little biology fact though is that the fish usually only go after the "adult" when there is a low food supply or a short feed season. Genetically, however, they will bypass everything to get the emerger just after it shucks. Floating flies are nothing more than "debris" drifting by. The same fly can be twitched just under the surface and become a "struggling emerger" that cues the genetic feeding response in the fish.
BTW, ever notice how many fish hit your fly just as it goes under the surface as you pull on it for the next cast, on still water, or hits the end of the sweep and you've made 3 or 4 tugs recovering line for the next cast? The struggling emerger is why, and no, fish don't have a brain, so, yes, you can use dry flies wet and actually get more results.