Walk a freestone stream and you'll see the same three water types repeating: riffle → run → pool, riffle → run → pool. They alternate because of how flowing water erodes and deposits its bed. Knowing which one you're looking at tells you where the fish are and how to fish it.
Riffles are fast, broken water spilling over rocks. The surface bubbles and breaks. Riffles oxygenate the water and dislodge insects — feeding factories. Fish don't sit in riffles (too much current to hold), but they sit at the top and bottom, picking off whatever drifts past. Best fished with a short cast and a drift-along-the-bottom nymph rig.
Runs are the in-between water — fast enough to dislodge food, slow enough for fish to hold position. Knee-deep, broken-but-consistent surface. This is where dry-fly fishing peaks during a hatch. Fish hold mid-run, eating from clear feeding lanes. Cast upstream, dead-drift past, and watch.
Pools are the deep, slow stretches at the end of a run. Big fish live here — they retreat to pools to rest, conserve energy, and avoid predators. Don't waste time fishing the middle of a pool: the holding fish aren't actively feeding. Cast the head (where the run dumps in) and the tail (where the pool spills back out). Both are seam water.
Most days, you'll find catchable fish in the run. Pools hold the fish-of-the-day; riffles hold numbers. Runs are where you'll usually be casting.