Which term describes a fast-flowing and turbulent part of a river?

The term that describes a fast-flowing and turbulent part of a river is "rapids." Rapids typically occur in areas where the riverbed has a steep gradient, leading to increased water velocity and turbulence as water flows over rocks and other obstructions. This results in churning and foamy water, often creating a chaotic appearance and sound.

In rivers, the formation of rapids is a direct indicator of the river's energy and slope, often found in mountainous regions or in the upper course of a river system. Other terms such as "bends" refer to the meandering sections of a river, which do not necessarily indicate fast flow, while "falls" usually refer to waterfalls, where water drops vertically rather than flowing rapidly horizontally. The term "ahead" does not apply in this context. Thus, rapids are specifically associated with the dynamic and turbulent flow characteristics of a river.