These should be recognized and fished by all fishermen. Riffles are fast, shallow and choppy parts of the stream. The faster water hitting rocks creates them. The rocks obstruct the current creating a pocket of slower water. Runs are the slower section of water that are usually below the riffles.
Runs are what fisherman refers to as classic dry fly water.
Runs are usually about 4 feet deep and much easier to wade then the riffles. Another important characteristic of the run is the center current. The center current is referred to by the fisherman as the scum line. This can be visible to the fisherman because of the presence of bubbles and insects being carried downstream by the stronger current. Fish will often hold in the deeper water and use the center current or scum line to feed.