By far the best method of covering big rivers is drifting them. If the river is huge, and laws allow one could also use a pwerboat, but unless you are with an experienced guide or know the river well yourself, you are likely to miss many a fine fishing opportunity speeding passed them. Drifting allows you to get a feel for the river, allows you to observe them diligently, learning the niuances of the specific river and the fish that inhabit it. If the water is clear, you can observe them right beneath you, the type of water they concentrate in, how easily spooked they are and their size. You can watch for risng fish, as well, and the water they prefer when actively working the surface. All of this is hard to see in a power boat, and it is hard to cover as much ground on foot. But one can be an even more carfeul observer on foot.
Breaking up the river:
The big rivers need to be broken up, at least mentally in order to successfully fish them. It is better if they are broken up topographically as well. Islands are by far the easiest feature for anglers to recognize. An island provdies a break in the current, and a natural place for gravel bars, tailouts, riffles, and pools. Islands in short turn the big river into two smallers ones, temporarily. Provding bug and fish habitat and if you are drifting, they provide a less prssured fishing spot, in theory. Although if your river is heavily dirfted, islands are apt to recieve a lot of fishing pressure.
Change in current and river depth, also provide natural fishing spots. A river that has been churning slowly will likely hold fish right above a slight drop, as the current begins to speed just slightly, fish are likely to lay in great abundance above a rapid. Likewise if the depth has been deeper than normal say 20-30 feet, and then turns to gravel, or smaller sized rock bed, fish also are liely to gravitate.
Also look for close in seams in the river. A seam is an area where the water is moving slower then the water farther out. Fish love to layin seams, and feed on the nymphs that heve been carried down by the faster moving water. If you can find a seam on an inside cirve in the rier all the better.
Rocks and logs also provide a fish a natural barrier. A place to hide, and a place to feed. Small pools created by large rocks are often an overlooked hiding spot for larger than average fish, that are actively feeding. They are worth a few casts, and the rewards are worth the casts that didn't turn anything up.
Other natural places to cast
Fish in big rivers are used to dealing with predators. Not just anglers, which they have to deal with as well. For that reason they need to feel secure. They love hind in undercuts from the bank and/or islands. Deep water right next to the bank is often best fished from downstream and csating up. This will minimize your chance of spooking them. Anytime I find water three foot or more in depth huggung the bank, I give it a couple of casts.
Also look for fish under tree limbs. This is by far the most frustraiting fishing you will have on the big rivers. Normally the provide ample casting room, and are generous with casting inaccuracies and back casts. But fishing under trees takes some patience, some expertise, and some luck. Often roll casts are called for, often an ability to read the microcurrents is called for, and allow the irver to deliever your fly to feeding fish. Stealth is the name of the game here, but if you make it a habit to practice, in no time, you will be hooking some very large trout.
Rigging up:
Big rivers not only are home to larger fish, they are also home to swift currents, and lots of room for fish to run. Stouter than normal rods are recommended if not required. For trout use at least a 6 weight, you will be glad you did. It will also afford for longer casting. Stouter tippet is also needed, and a reel full of backing can be the difference between landing that fish of a lifetime or breaking it off. Split shots or Xink will be needed if fishing sub-surface, as fish are likely to be hugging the bottom.
One last word is on safety. Big rivers can be very swift, and very difficult to read. They can also have vicious drop offs. Wading staffs, and cleated or at felted wading boots are a must. If in doubt stay on the bank. Plenty of great fishing is usually only a few feet off from the edge.
About the Author
Cameron Larsen is a retired commericial fly tier and fly fishing guide. He now operates The Big Y Fly Company. http://www.bigyflyco.com/flyfishinghome.html He can be reached at info@bigyflyco.com. This article will appear in the Big Y Fly Fishing E-Zine at Http://www.bigyflyco.com/Bigyflyfishingezine.html