Reading a river and identifying lies is one of the most important skills you can possess as a fly fisherman. If you can’t recognize spots where fish are likely to be holding you won’t cast to those spots and subsequently won’t catch any fish.
Here is a quick case study.
This is a very basic structure you’ll see on just about any freestone waterway in the world. A partially submerged rock is splitting the current into two separate runs.
This is a good spot for fish to hold as the area behind the rock is quite deep (in this case roughly 3′), the rock gives them shelter from the current, and the current carries food right to them. In this instance there was also a bit of structure in the form of an underwater rock overhang in the run on the far side. This is where the fish were holding and I pulled two from this one spot.
When fishing a spot such as this, cast your fly up stream of the rock and let it drift down through the runs on either side. With any luck you’ll find a hungry trout waiting for your fly.