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.
Pools are generally easy to distinguish because they are the deepest parts of the stream.
They get their name because they are often used as swimming pools. Pools are usually six feet deep or deeper and are great for fish to hold. They are usually fished with weighted nymphs and streamers in order to get the flies down to the fish. When hatches of Mayflies are thick enough the fish in these pools will come up and feed on the surface. Flats are large slow moving sections of the stream.