thumb|Rapids of [[Kern River, California, USA.]]

Rapids are sections of a river where the river bed has a relatively steep gradient, causing an increase in water velocity and turbulence. Flow, gradient, constriction, and obstacles are four factors that are needed for a rapid to be created.

Physical factors

Rapids are hydrological features between a run (a smoothly flowing part of a stream) and a cascade. Rapids are characterized by the river becoming shallower with some rocks exposed above the flow surface. As flowing water splashes over and around the rocks, air bubbles become mixed in with it and portions of the surface acquire a white color, forming what is called "whitewater". Rapids occur where the bed material is highly resistant to the erosive power of the stream in comparison with the bed downstream of the rapids. Very young streams flowing across solid rock may be rapids for much of their length. Rapids cause water aeration of the stream or river, resulting in better water quality.

For a rapid to form, a necessary condition is the presence of a gradient, which refers to the river or stream's downward slope. When a river has a larger gradient, the water flows downhill faster. Gradients are typically measured in feet per mile or metres per kilometre. This impacts the river's flow or discharge, which is measured as a volume of water per unit of time. The faster the water flows, the more likely a rapid will form.

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File:Rapids before the Rhine Falls.jpg|Rapids featuring whitewater, close to the Rhine Falls

File:Violent water below Niagara Falls.jpg|Violent water below Niagara Falls

File:RapidsonMississippiRiverOntario.jpg|Rapids on the Mississippi River (Ontario) in Pakenham, Ontario, Canada.

File:Rapids on Maumee River above Toledo, Ohio - DPLA - b4406d60cafd34da9ef2473273e08076 (page 1).jpg|Historical image of the river rapids on the Maumee River in Ohio

File:Uusimaa, Finland - panoramio - pan-opticon (22).jpg|The Kuhakoski rapids in Uusimaa, Finland

File:Storforsen-17.JPG|Storforsen, rapids along the Ume River, Norrbotten, Sweden

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See also

  • Fluid dynamics
  • International scale of river difficulty - for classification of rapids
  • Rheophile - organisms that live in fast-flowing water
  • Riffle - A fast-moving portion of a stream without the vigor of a rapid
  • River rapids ride

References

  • Rapids entry in National Geographic's encyclopedia