River rapids exist because hard rock refuses to wear down as fast
Rapids form where a riverbed tilts steeply and the rock beneath resists erosion better than the stretch downstream. Soft layers wash away, hard ones stay, and water tumbling over the resulting steps traps air and turns white. That churning also leaves the river cleaner, because the water becomes richly aerated.
Hydrologists place rapids between a run, where a stream slides smoothly, and a cascade. The water gets shallower, rocks break the surface, and as the flow splashes around them it mixes in bubbles that whiten patches of the surface, the familiar whitewater. A gentler fast-moving section without that vigour is called a riffle, and organisms adapted to such torrents are known as rheophiles.
Four ingredients are needed: flow, gradient, constriction and obstacles. Gradient, the downhill slope, is measured in feet per mile or metres per kilometre; the steeper it is, the faster the water runs and the greater the discharge, the volume passing per unit of time. Constriction squeezes a river into a narrower channel and speeds it up, which can also pile up sediment into new obstructions. Obstacles may come from landslides, earthquakes, accumulated debris or human activity. The more strongly all four are present, the more likely a river is to churn.
The underlying mechanism is differential erosion. Where sloping strata of different hardness make up the bed, softer rock is scoured away quickly while harder bands persist, leaving an uneven, stepped floor. Very young streams cutting across solid bedrock can be rapids along most of their length.
For paddlers, sections are graded on an international scale from Class I to Class VI according to how navigable they are. Class I needs little steering. Class V can be split finely from 5.1 to 5.9. Class VI endangers life with little hope of rescue and is often marked U, for unraftable. Rafting as a sport concentrates on rivers with many rapids along their course.
Source: Rapids
