Understanding current will be a vital to learning how to read water and determine where fish hang out. Fish are always in motion. Their survival depends on a positive balance between energy expenditure (-) and energy conservation (+). With this in mind, it makes sense that trout are usually found where:
1. Food is abundant
Healthy rivers and streams offer a variety of food sources (Midges, Mayflies, Caddisflies, Stoneflies, Terrestrials, Leeches, Crayfish, etc.). throughout the entire year. If the water you are fishing doesn't have much food, it probably doesn't have many fish.
2. Minimal effort is required
A trout's overall goal is to find a spot in the river where food is brought directly towards them, while exerting as little energy as possible to stay in place. This is where current comes into play...
Current varies from bank to bank and surface to riverbed.
Bank to Bank
Rocks, other debris, and bend in the river create an area known as a seam. This is where fast current and slow water meet (see picture below). Trout will hang out in the slower water near the edge waiting for food in the current. When a substantial meal passes by, they will zoom over, snag the food, then return to the slower water.