For the card shuffle technique known as "riffle", see Shuffle
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A Riffle is a short, relatively shallow and coarse-bedded length of stream over which the stream flows at higher velocity and higher turbulence than it normally does in comparison to a pool. As a result of the increased velocity and heightened turbulence, small ripples are frequently found. Riffles are usually caused by an increase in a stream bed's slope or an obstruction in the water.
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 Hjulstrom 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.
Gold mining 
The term is also applied to strips of wood or metal laid across the washing tables during gravity separation of gold in alluvial or placer mining.
See also