Rapids are areas of shallow, fast-flowing water in a stream.

Rapids tend to form in younger streams, with water flow that is straighter and faster than in older streams. Softer rocks in the streambed erode, or wear away, faster than harder rocks. This process is known as differential erosion. The result of differential erosion is that as the streambed wears away, the stronger rocks remain and eventually begin to break up the flow of the stream. The many tiny waterfalls they create make the slope of the stream more steep.

The safety of a section of river is measured by the class, or level, of its rapids. The class of a rapid determines how difficult it is to navigate using a kayak, raft, or other vessel.

  • Class I: Small waves, no obstacles.
  • Class II: Medium waves, no obstacles.
  • Class III: Many waves of different strengths, many obstacles, narrow passages.
  • Class IV: Many strong waves, many dangerous obstacles, whirlpools.
  • Class V: Constant strong waves, constant obstacles, whirlpools, fast currents, some waterfalls.
  • Class VI: (also classified as U, for "unraftable") Constant strong waves, constant obstacles, whirlpools, fast currents, steep waterfalls.

Rapids can be important to the health of a stream system. The water splashing over rocks captures air in bubbles. This splashing, called whitewater, leads to more dissolved oxygen in the water. The oxygen is useful to fish, insects, and bacteria in the water, and in turn to the ecosystem around the stream.

rapids
Rowboats are not the safest way to navigate rapids.

Unpredictable Rapids
Many rivers have different stretches with different classes of rapids. The class of rapids can also depend on the weather. A monsoon or drought can increase or decrease the class of rapids.

The Wang Thong River in Thailand, for instance, has slow-moving, smooth stretches of Class I rapids as well as wild, fast-moving whitewater rapids that are often Class V.

Plural Noun

(singular: bacterium) single-celled organisms found in every ecosystem on Earth.

Noun

steady, predictable flow of fluid within a larger body of that fluid.

determine
Verb

to decide.

differential erosion
Noun

the process of softer rocks wearing away faster than hard rocks.

Noun

period of greatly reduced precipitation.

Noun

community and interactions of living and nonliving things in an area.

erode
Verb

to wear away.

insect
Noun

type of animal that breathes air and has a body divided into three segments, with six legs and usually wings.

Noun

seasonal change in the direction of the prevailing winds of a region. Monsoon usually refers to the winds of the Indian Ocean and South Asia, which often bring heavy rains.

navigate
Verb

to plan and direct the course of a journey.

obstacle
Noun

something that slows or stops progress.

oxygen
Noun

chemical element with the symbol O, whose gas form is 21% of the Earth's atmosphere.

Noun

areas of fast-flowing water in a river or stream that is making a slight descent.

Noun

large stream of flowing fresh water.

steep
Adjective

extreme incline or decline.

stream
Noun

body of flowing fluid.

streambed
Noun

material at the bottom of a stream.

Noun

flow of water descending steeply over a cliff. Also called a cascade.

whirlpool
Noun

llquid flowing quickly in a circular motion, pulling material downward into its center.

whitewater
Noun

fast-moving parts of a river.