After learning about the trout itself, you should learn about rivers. You'll see that it's easy to understand where you'll find the fish if you understand the anatomy of a river. Luckily for us, it's not that complicated. If the trout's figured it out, hopefully you'll be able to.
River currents tend to flow with a pattern that repeats itself over and over. Just remember: "riffle-run-pool" [source: Troutlet]. Here's a basic breakdown of each section and how it translates to trout fishing.
The riffles of rivers tend to be where water is shallow and the current is strong. In large rivers, this area would be the white-capping rapids. In all rivers, you might see banks of gravel or pebbles breaking the surface of the river throughout the riffle. For the most part, this area of the river will only contain small trout, because the water isn't quite deep enough to ensure large fish cover.
The run of a river is deeper and slower than the riffle. If you're looking for the area of the river that house the most adult trout, this is your best bet. It provides good cover and the current is a great moderate speed that allowed the lazy trout access to a sufficient buffet. After all, the trout's ideal habitat is one that provides him with adequate shelter and delivered food.
River pools are the laziest part of the river. Here the water is deep and the current runs slow. Some trout, especially big Brown trout, may be found here, but the slow current doesn't provide enough food for most.
Now that you understand the current and its effect on trout, read on to the next section to learn how to use that knowledge in fly-fishing.