Wetnet,
The fishing is a bit more difficult on big water because it adds the element of locating fish to the equation. While it's not hard to locate fish on a stream as wide as your rod is long, a large river poses a challenge, since many times the fish, at least the active fish, will usually be located in certain water types and absent in others.
The key to being successful on a river is to locate the “active” fish. To explain, let’s use a typical pool as an example. In the deepest part of pool, a ton of fish may be holding near the bottom but not actively feeding. The main pool may hold the most fish, but these fish are usually tough to catch. Perhaps some might be coaxed into hitting a fly, yet at the head and tail of the pool, actively feeding fish may be ready to pounce on your fly with reckless abandon.
Don't become overwhelmed by all the water. Break the river down into small sections and fish it methodically, the same way you would fish a smaller stream. Don't overextend your fishing area and try to fish too large section of river a one time.
Having said that, one way to start your search is to fish a streamer or bugger and cover the water thoroughly and quickly. You may hook some fish, or at the very least, have some misses or follows, which will help you locate some of the spots that hold active fish.
Search different water types, pools, riffs, runs, etc. Within those water types, concentrate on current seams, obstructions, depth change areas, etc. At some point you will begin to catch fish (hopefully) or get hits. Take note as to where the fish are holding such as depth, velocity, cover, shade, etc., and concentrate on similar areas in the river. Most of the time, except during extreme conditions, there will be active fish on some part of the river. It’s up to you to locate them.
After fishing certain river, from experience, you will know which areas to concentrate on that will most likely hold active fish. With some experience, even on unfamiliar water, you will have a good idea as to where the active fish may be, and where to start your search.
Good luck.
