Rivers, when you get it right, are generally ‘easier’ to extract trout from, as you have a couple of things working in your favour.
1) Assuming there’s nothing actually rising and taking insects off the top, it’s not too difficult to work out where the fish will be in moving water, to target them with sub-surface patterns that imitate the ‘nymph’ stage of an aquatic insect.
Remember that most aquatic insects spend the best part of a year in this stage, only to emerge into a ‘fly’ to mate and lay for the last day of its life, so in rivers, sub surface is very much where the food is.
Trout like an easy life, and will sit in the spots that offer them the most amount of food while they expend the least amount of energy. They will sit behind rocks, in front of rocks, and in the slightly slower areas along side the currents that conveyer dislodged aquatic life along the river. You don’t have that luxury on a stillwater, there will be the odd cruising fish, but in general stillwater fish tend to home in on localised hatches and movements of midge larvae, and these can be anywhere, and depending on the stage of the hatch, at any depth.
2) Moving water : in a river where a trout is holding a position to intercept items of food that pass it by, they often don’t have time to closely scrutinise what they perceive to be a food item, and if it ticks enough ‘ looks like food ‘ boxes, it will take it, but very quickly eject it if it senses what it has taken is inedible. In a stillwater, this for the most doesn’t apply, as the fish have time to inspect your offerings, and you’ll often see them turn away at the very last moment. This can be down to the look of the fly itself, how it sits in the water, or partially with surface patterns, your line/leader.
Likewise, on a river, the water currents can very much work against you, especially with surface patterns. If your dry fly is not moving at exactly the same speed as other tiny flotsam moving along, they’ll not take it. To ensure your presentation is correct means being very careful how your main fly line and leader is sat in the water- fast currents acting further up your line can cause the fly in the end of the line to skit about in an un natural manner. The only exception to this rule is when fishing certain patterns at certain times - ie, when hatched flies such as sedges that behave erratically anyway are present on the water.
The one thing though, that is common across both rivers and stillwaters , relating to fish activity, is environmental conditions. Ideal conditions for most insects to make the transition from river bed to flight, is warm, dim, and breezy, and you’ll generally see most surface fish activity in these conditions.
Regarding tackle, rivers in particular, your choice of rod length and line weight is very much governed by the style of fishing you intend to do, and the size of river you intend to fish. I haven’t fished a river for 4-5 years now, but really only needed 2 rods. For medium to large reasonably pacy rivers, a 9’6”#4 is a good all rounder for dry/nymph/small streamer ( lures ).
For smaller rivers and brooks, I would use something around the 7’6” #4 range for the same techniques, but on smaller brooks it would mostly be quick evening sessions for dry fly.
Regarding tuition, if I were you I would try and get yourself into a position where a friendly fisherman that knows the ropes will take you out for the day, and you share a rod - you’ll just need waders. Guides and instructors are all very well, but your money will be better spent once you have the basics already covered, such as wading, reading the water, fly selection etc.
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