It is interesting to see the difference that location and species can have on an angler!
I cut my teeth and grew up fishing streams for trout just like you described for about 10 years. Moving out to the midwest and fishing for bluegill and bass has been a big curve ball for me. I can definitely relate to what you're talking about!

Here are a couple of my initial thoughts that might start you in a good direction.
*If you're fishing a river that has an abundance of natural food, your flies and presentation has to be spot on. And I mean exactly spot on perfect 95% of the time. (the other 5% you just get lucky)
*If you're fishing a river where fish rely on terrestrial activity for a lot of their food source you can generally get away with tossing any buggy looking dry fly. Think skinny water and small spring creeks here.
*Fish pocket water!! It sounds like you already are with this line: "On the river however it just kept getting away from me. I wondered how the fish would every see my fly with it shooting down the river so fast..." Use this to your advantage. The good thing is that the fish have less time to react to your offering. If it is close to what they're looking for, they better eat it or else miss out on precious nutrition. (as opposed to fishing still water for gills and bass where the fish have time to call their doctor in to inspect the fly with a microscope, take cultures, run a blood and DNA test before deciding whether to eat it or not).
*If you do happen to be fishing s deep river you'd better use tungsten beads and/or split shot to get you nymph down to where the fish are.
*Don't be afraid to go small!! If you're used to fishing for bass and bluegill think one or 2 hook sizes smaller than your initial thought would be (almost all of my trout nymphs are smaller than a 12 with most being a 16).

Hope that helps. PM me if you're looking for longer explanations