An alternation between a deep zone (the pool) and a shallow zone (the riffle) along the sand and/or gravel bed of a stream. The pool-to-pool spacing is about 5–7 times the width of the channel. The sequence is found in both straight and meandering channels; in the latter case the pool occurs in the meander bend on the concave side, and the riffle between bends.

Pool-and-riffle