Reading Your Stream
From time to time I get the question - "what flies should I use on my stream?" from places that I've never been nor fished. The only answer I can really offer without more information is to suggest starting with the basic nymphs, dries, and streamers and experimenting from there. When I fish in a new spot I do some investigative work first by internet and/or phone but, upon arrival, by looking at the water and reading the signs. Here are a few things to try to be able to select a reasonable fly and hopefully get you into fish faster.
Look for fish. If fish are at all visible see what they are doing. Rising fish are easy to spot. If they are rising go with a dry fly or an emerger. If they are visible under water and are moving quickly from side to side in a feeding manner then drift a nymph to them. If they are visible and sitting still they may not be feeding. Try a nymph, then maybe a streamer pulled across current in front of them.
Look for flies. If flies are visible floating on the surface and fish are rising - jackpot - match them in size, shape, and color in that order. If flies are visible on the surface and no fish are rising then determine what kind of flies they are and find a nymph that is an appropriate match for the nymph of that species and match it in size. An entomology guide book can help you match the nymph to the adult. If no flies are visible on the surface start turning rocks and determine what size and type of nymphs are most prevalent and match them. You should also try to use a seine to determine which insects are actually drifting.
A very general but good rule of thumb for trout streams is that in cold water and tailwaters go small, size 22-16. In freestone streams go bigger, size 16 - 10. In fast pocket water use a little larger and visible attractor patterns. In smoother, slower water match the bugs as closely as possible with usually a smaller fly.
Fly fishing is an amazing but very enjoyable exercise in problem solving. Use these techniques to read your stream and find the right fly, then, well rigged anyway, you can work on the other problems of approach and presentation.