"Rapids are sections of a river where the river bed has a relatively steep gradient, causing an increase in water velocity and turbulence.
Rapids are hydrological features ...
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between a run (a smoothly flowing part of a stream) and a cascade. Rapids are characterised 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 colour, 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." - (en.wikipedia.org 01.12.2019)
