What is a characteristic feature of rapids in a stream?

The characteristic feature of rapids in a stream is swiftly flowing water over rocks. Rapids occur in areas where the gradient of the stream bed is steep, causing the water to accelerate as it flows over rocky sections. This rapid movement creates turbulence and a foamy appearance, which is indicative of high energy flow. The presence of rocks and boulders disrupts the smooth flow of water, resulting in fast currents and whitewater conditions that are typical of rapids.

In contrast, slow-moving water represents sections of a stream where the gradient is gentle, leading to a more subdued flow. Still water is found in areas like pools or lakes within a river system and is characterized by a lack of significant movement. Increased sedimentation can occur in slower flowing areas where the water has less energy to carry sediment, which is not synonymous with the feature of rapids. Thus, the defining quality of rapids is indeed the swiftly flowing water over rocks.