If you are interested in catching trout in a rapidly flowing river or stream, a common strategy is to seek out a pocket of slower moving water where the fish congregate to feed and conserve energy. Stream pools are often deeper than the main part of the river offering the fish additional cover from predators such as eagles. Of no particular benefit to the fish, the slower water also makes it easier for the angler to stand in the river for extended periods, tying flies and casting repeatedly without the threat of being swept away by the more powerful flow in the middle of the river.
This shot was taken on the Gros Vente River in Jackson Hole, Wyoming. Although the picture creates the impression that I know how to catch a fish, we actually went away empty-handed from this particular spot (and many others). Even though we could see large cutthroats gorging on aquatic insects only a few feet in front of us, we spent an hour unsuccessfully tying and re-tying different flies frantically trying to match whatever unlucky critter the cutts were finding so scrumptious. Entomological ignorance proved to be our undoing.
Surprisingly, one of the pleasures of flyfishing is discovering and puzzling over a stream pool even if it doesn't result in catching a trout -- which, if caught, should be released anyway. Departing from a stream pool, you are always empty-handed, retaining only the memory of a shared sacred space and an intimate encounter with life's ingenious way of coping with a powerful and fast-moving flow of water.