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.