pool and riffle
Few things assert the dynamism of nature better than the development of pools and rifﬂes in a stream. Even if a channel is straight and the bed uniform, ﬂowing water will generate turbulence. Depending on the resistance of the streambed, this turbulence may scour pools in certain places, which alternate with shallow bars called rifﬂes or drops, where the excavated material is deposited. Generally the distance from one pool to the next is ﬁve to seven times the width of the stream, and successive pools will tend to develop on opposite sides of the channel, precursors to the formation of meanders. Very steep streams have step-pool sequences with similar cyclicity. Experienced anglers tend to be attentive to the patterns of pools and rifﬂes, as ﬁsh like to wait in the slow water of a pool while watching for food to arrive from the faster ﬂow of an upstream rifﬂe.