A rapid is a section of a river where it loses elevation over a relatively short distance (that is, the stream gradient is locally steepened), causing an increase in water flow and (usually) turbulence. A rapid is a hydrological feature between a run and a cascade. A rapid is recognized by the river becoming shallower and having rocks exposed above of the flow surface. As flowing water splashes over and around the rocks, air bubbles become mixed in with it and the water takes on 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.

Classification of rapids in whitewater sports

The sports of canoeing and kayaking have developed grading systems for rapids (Caution: these can differ somewhat from country to country). Rapids are classified as follows:

• Class 1 - no rapids, smooth flowing water
• Class 2 - some rough water
• Class 3 - whitewater but very safe for larger rafts
• Class 4 - whitewater for experienced rafters only
• Class 5 - whitewater for experts only
• Class 6 - impassable

See also rafting for a rafting grading system for rapids.

Even with such systems, the meaning and understanding of the word rapids, can vary enormously, depending on the perception, skill level, experience and bravery or foolhardiness of the paddler.

Also, some rivers with huge volumes of fast moving water may require little manouevering, but will pose serious risk of injury or death in the event of a capsize. Other, more technical rivers, may require great skill in negotiating through many rocks, but with low water flows may pose only a low degree of danger.

The grading system in Australia is as follows:

1. Small regular waves. Easy passage, but care may be needed with obstacles.
2. Regular medium sized waves and generally unobstructed passage.
3. Fairly high waves, and the passage may be difficult to recognise from on the river.
4. High, powerful, irregular waves, with the passage often difficult to recognise.
5. Very difficult rapids; the extreme for commercial operations.
6. Very dangerous, and at the limit of practicality.

See also

• whitewater
• wild water