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.
They are very shallow and generally the same depth from one side to the other. These areas do not hold many fish but are ideal places for fish to feed when hatches are on the water. When sufficient hatches are on the water, fish will pull out of the deeper waters onto the shallow flats. Here they can easily locate the insects and feed on them without using much energy. However, with the shallow slower moving water, fooling these fish into taking your fly can be difficult.