What do we call the part of a stream where the water flows very swiftly over rocks?

The part of a stream where the water flows very swiftly over rocks is referred to as rapids. Rapids are characterized by turbulent and fast-moving water, which occurs when water flows over uneven surfaces, such as rocks or steep gradients. This rapid movement creates features like waves and eddies, contributing to the dynamic ecosystem of the stream.

While other terms also relate to different aspects of stream environments, they do not specifically indicate swift water over rocks. For example, the current refers to the overall flow of water in a stream but does not specifically highlight the rapid or turbulent nature found in rapids. Riffles, on the other hand, represent sections of a stream where water flows over rocks in a shallower area, but they are generally calmer than rapids and may not exhibit the same level of turbulence. Streambed pertains to the bottom of the stream itself, where water flows, but it does not define the nature of the turbulence or speed of the water above it.

Thus, rapids distinctly denote areas of swift water over rocks, capturing the essence of a specific and dynamic feature within stream ecosystems.