In a naturally flowing river, areas of riffles and pools alternate in a predictable sequence and at predictable intervals which are related to the width of the stream. (Riffles are where shallow water is rippling over rocks; pools are deeper and calmer areas) Determining the riffle-pool sequence and stream width can give students an indication of the extent to which the stream they are in is out of equilibrium with the forces that create it. In most streams the riffles are found a distance apart that is equal to 5 to 7 times the width of the stream.
Determining the riffle-pool sequence is an exercise that only requires walking along the stream with a tape measure and a notebook.
From earlier discussions on the life of brook trout we know that both the riffle and the pool are needed by the trout. The riffles are where most of the invertebrates that form the food of the younger trout live and the trout often wait at the bottom of riffles for food to wash down to them. The deeper pools with their slower water provide protection and resting sites for the trout. In the winter these deep pools may be the only place where the river does not freeze all the way to the bottom and because of this the deep pools are essential for the trout's overwinter survival.