Riffle

A riffle is a shallow section of a stream or river with rapid current and a surface broken by gravel, rubble or boulders.

Riffles are instrumental in the formation of meanders, with deeper pools forming alternately. Although simple fluid flow suggests slower flow in deeper water and faster flow over riffles, the true flow pattern pool and riffle waters is often helicoidal flow or turbulent, which permits more rapid erosion of the wetted perimeter. Nevertheless, the coarse-grained bedding of riffles suggests erosion of smaller particles, according to the Hjulström curve. Riffles are typically found in the middle course of rivers, and are theoretically found at intervals around 6 times the width of the river, although local conditions cause this to vary.

In the fishkeeping world, a "riffle tank" is one specializing in aquatic life that originates in places with powerful currents like riffles. These are usually emulated with very powerful pumps.

The term is also applied to strips of wood, metal or plastic laid across the washing tables during gravity separation of gold in alluvial or placer mining.

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