In fact, moving water striking an object such as a rock actually forces water to curl back, or back draw, behind the rock. The larger the boulder, typically, the bigger the fish using it as refuge. Huge rocks or the combination of several boulders in sequence might house a school of fish. Another thing to consider about rock structure is that some boulders do not break the surface. Boils and mysteriously back curling water generally reveal the presence of large underwater rocks.
Pools formed along the riverbank are other popular fish haunts.
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.