For instance, windblown trees entering the current cause a pool to form directly behind them. Over a period of time floating debris collects on the windfall, ultimately constructing a natural dam. The back curling water effect then actually excavates bottom content from behind the new dam making an even deeper pool. The size of the dam combined with current speed tends to dictate pool size. And pools are one of few locations on a vibrant river that contain a silt bottom.
Soft bottoms are important to species like catfish, and insect eaters such as crappies and sunfish.
Another common location to find a pool is at the tailrace or end of shallow rapids. Fast moving water over rocks or gravel either tapers downward or ends abruptly into a pool. These pools vary in depth and they often appear in narrowing stretches of a river. Tailrace pools collect plenty of debris and baitfish, and there's no better locale to encounter highly active fish. Shoreline eddies can be caused by any and all of the aforementioned structures.