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 lower 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
Helicoidal flow
Helicoidal flow is the cork-screw-like flow of water in a meander. It is one example of a secondary flow.Helicoidal flow is a contributing factor to the formation of slip-off slopes and river cliffs in a meandering section of the river...

 or turbulent, which permits more rapid erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 of the wetted perimeter
Wetted perimeter
The wetted perimeter is the perimeter of the cross sectional area that is "wet." The term wetted perimeter is common in Civil Engineering, Environmental Engineering and heat transfer applications; it is associated with the hydraulic diameter...

. Nevertheless, the coarse-grained bedding of riffles suggests erosion of smaller particles, according to the Hjulstrom curve
Hjulstrom curve
The Hjulström curve, named after Filip Hjulström , is a graph used by hydrologists to determine whether a river will erode, transport, or deposit sediment. It was originally published in his doctoral thesis "The River Fyris" in ?...

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.

Gold mining

The term is also applied to strips of wood or metal laid across the washing tables during gravity separation of gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

 in alluvial or placer mining
Placer mining
Placer mining is the mining of alluvial deposits for minerals. This may be done by open-pit or by various surface excavating equipment or tunneling equipment....

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