While some trout live out their entire lives in basically the same spot, many trout migrate either to spawn or because of natural environmental needs. The most obvious of these are sea-run or lake-run trout like steelhead or various other species that have a similar life history. Some other trout, like some cutthroats in parts of Yellowstone Park, will migrate according to seasons and environmental changes in the rivers they live in. Here are a few things to think about when targeting migratory trout.
Follow the fish. First get all the info you can on the progression of runs before going out. Since these trout are migratory, you may need to chase them and might not find them in the same spot for long. Be agile and ready to move up or down river if nothing is happening. However, remember that migratory trout don't feed as much during migration and so can be tough to catch and may require a number of casts to finally entice or provoke a strike. Move when nobody around is catching anything, but if a few are, then pound the water until you also hook up.
Look for natural barriers to migration. Often migrating fish traveling upstream will be stopped and will be found in much greater numbers just downstream of either natural or man-made barriers like falls, diversions, or dams. Fishing below such structures can be fast and furious but fishing above can be a total waste of time.
Fish fly patterns that imitate familiar foods. Migrating trout can often remember their favorite food sources from the lakes, rivers or ocean they are coming from. Trout know and remember the prey they have eaten most of their lives, even if it isn't stuff that is found in the rivers where they are currently found. Try fishing leech, scud, and chironomid patterns to lake run fish or patterns that imitate bait fish like clouser minnows to sea run fish.
High flows move fish. Most of the time when rivers go up because of rain or other factors so will running fish like steelhead and salmon. If you get a bunch of rain remember that the fish will move upstream more during these periods than during periods of low water.
Fish staging areas as well. Remember that many times trout, like salmon, will stage at river mouths either in salt water or in fresh water lakes just before running upstream. If you seem to be just a little early for the run try fishing the mouth of the river with streamers, buggers, or leeches.
Fall is a great time to get out and catch steelhead, lake-run rainbows, lake-run browns, and various other migratory trout. Be ready to hit the runs near you soon.