You'll probably get as many opinions as responses for a question like this
You may catch a single large trout in streams that don't have the type of habitat that you think would generally produce big fish; they are usually migratory fish and probably not long term residents.
When I think of the few streams that I know of that consistently have large trout in them, the common component is an abundance of deep holes with a good amount of cover. There is also an abundance of bait fish - sculpins, white suckers, dace, etc. and most larger fish will shift over to a meatier diet. That doesn't mean they can't be caught on a trico imitation, but I'd say their primary diet is larger food stuffs - hoppers, crickets, mice, bait fish, baby/juvenile salmo truttas and baby/juvenile savelinus fontinalis. Deep hole creation is enhanced by the presence of woody debris, or often time single large rocks (like small house size rocks). Another type of cover that is often presence is deep undercut banks. Sometimes, these will go back in three to six feet and there's often a ball of roots, that serves almost like a jail that keeps out predators. I matched wits with a wily brown in such a setting for a few years running, before finally catching the fish; in the process of steering the fish out of his lair when I finally caught him, he kicked out a fish that was only about 3" his junior, which attested to how much space there must have been under there.
The other class of large trout habitat is, as Pete notes, is lakes or larger streams, and the fish that live there will move into smaller tributaries, either under thermal pressure, or to spawn.
