• 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.

    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.

  • Term Part of Speech Definition Encyclopedic Entry
    bacteria Plural Noun

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

    current Noun

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

    Encyclopedic Entry: current
    determine Verb

    to decide.

    differential erosion Noun

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

    drought Noun

    period of greatly reduced precipitation.

    Encyclopedic Entry: drought
    ecosystem Noun

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

    Encyclopedic Entry: ecosystem
    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.

    monsoon 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.

    Encyclopedic Entry: monsoon
    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.

    rapids Noun

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

    Encyclopedic Entry: rapids
    river Noun

    large stream of flowing fresh water.

    Encyclopedic Entry: river
    steep Adjective

    extreme incline or decline.

    stream Noun

    body of flowing fluid.

    streambed Noun

    material at the bottom of a stream.

    waterfall Noun

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

    Encyclopedic Entry: waterfall
    whirlpool Noun

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

    whitewater Noun

    fast-moving parts of a river.