INFERTILE WATERS
Opportunistic feeders that can be easily prospected with larger, bushy/buggy dry flies. In these environments where aquatic prey items are relatively scarce. Generally smaller trout (typically) rise willingly to Humpies, Wulffs, Irresistibles, Bushy Parachutes, Elk Hair Caddis, or terrestrials.

FERTILE WATERS (Spring Creeks, Limestone Creeks, Tailwaters, “select” Freestoners)
Generally selective feeders that prey on a very wide variety of aquatic insects. Fly selection in these bug-rich waters usually has to match a very specific species of fly in a specific size, posture, silhouette, and stage of development. Familiarity with insect identification and observation of bugs in the drift is critical. These are the waters and fish that tend to be difficult, however the reasons surface feeding trout choose to ignore your dry fly can often have nothing to do with the fly. Your mere presence and/or lining can put trout on high alert, despite continued feeding, As Gary Borger states, on a scale of 0 – 10, drag is a 100; “Drag-free drifts catch fish” and “Dead flies don’t swim” are apt mantras. Often these seemingly selective feeders ignore your fly simply because it is not within one inch of their drift line, and sometimes it’s a matter of timing as they are not looking up and ready to eat. And there are times when your fly is correct in every aspect, other than the very subtle motions of the naturals that are impossible to duplicate. Finally, there are the maddening smutters feeding on the TDS hatch

And then there are those times your casting, mending, and line control are perfect, and motion is a non-factor. These are the times when fly choice needs to be dialed in. There are few secrets when it comes to selecting appropriate mayfly, caddisfly, stonefly, or terrestrial patterns.

In fertile, bug-rich waters, DFO anglers are faced with fly selections that must cover matching the hatch, breaking the hatch, headhunting individual fish, and prospecting. On the NY tailwaters where I fish, mayfly and caddisfly emergers rule during most hatches. Hatch breakers for me are typically beetles, spinners, large drake duns, and an old school fly I would never leave home without: LaFontaine’s Sparkle (Caddis) Emerger. These same patterns cover headhunting non-hatch risers; prospecting is almost always a beetle thing for me, and occasionally hoppers. If I were restricted to just three styles of dry fly on my waters, they would be biot body DHEs, Coffin flies, big lead-belly beetles, and sparkle emergers.

My 2 cents on a topic I am fairly passionate about.

BTW, great analysis on upstream v downstream approach. Your distinction between wade angling and drift boating was spot on. The only fishing from above I do is across and just slightly above with a negative reach cast. Approaching from directly above when wading also kicks up plumes of silt and debris that will put fish down.