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Stream Anatomy — Riffles, runs, and pools

A mixture of flows and depth provide a variety of habitats to support fish and invertebrate life. Pools are deep with slow water. Riffles are shallow with fast, turbulent water running over rocks. Runs are deep with fast water and little or no turbulence.

When a stream meets up with a huge fallen log, or a set of boulders, the water pours over the top. The vertical force of the water falling down on the other side will carve out a pool in the stream. Pools are favorite places for trout to hang out, and since the water in them flows a little slower, some other animals do well here, including mollusks (like clams and snails) and worms. One of the benefits to slow-moving water is that organic debris settles out into it. Another advantage is that you don't have to relocate to another area if the stream level starts to lower.

Where there are pools, there are riffles. These are shallow places where water runs fast and is agitated by rocks. Only animals that cling very well, such as net-winged midges, caddisflies, stoneflies, some mayflies, dace, and sculpins can spend much time here, and plant life is restricted to diatoms and small algae. Riffles are a good place for mayflies, stoneflies, and caddisflies to live because they offer plenty of cobbly gravel to hide in.

Close to any pool or riffle is likely to be a run, which merely describes a main body of water that runs smoothly downstream. Fishes, like minnows, too small to compete for pools often end up in runs.

Text on this page is adapted from Stream Biology, a web site developed by: Cristi Cave, B.S., Fisheries, 1998, School of Fisheries, University of Washington.