Often, streams suffering from cumulative watershed effects have a large percentage of flatwater habitats, such as glides and runs, and riffles. Pools often have filled in, and represent a small percentage of habitat types. Plunge pools are formed where water falls over a boulder or log. The falling water scours a deep hole where juvenile and adult fish often hide. Logs, root wads, boulders or stream banks can cause back water pools to form as water swirls around the obsticle.
Coho salmon almost always rear in pools formed by large wood.
References Bisson, P.A., J.L. Nielsen, R.A. Palmason and L.E. Grove. 1981. A system of naming habitat in small streams, with examples of habitat utilization by salmonids during low streamflow. N.B. Armantrout ed. Acquisition and Utilization of Aquatic Habitat Inventory Information. Hagen Publishing Co., Billings, Montana.