What part of a stream is defined as the area where water collects to flow downstream?

The channel of a stream is the part that specifically refers to the physical confinement where water collects and flows downstream. This is defined by the banks and the bottom structure of a stream, which directs the movement of water along a specific path. A channel can vary in width and depth depending on various environmental factors, including the volume of water flow and the landscape through which it passes. Understanding the channel is essential for studying aquatic ecosystems as it plays a crucial role in determining water flow dynamics, habitat availability, and sediment transport within the ecosystem.

While other options may relate to aspects of streams, they do not accurately describe the concept of a specific area where water collects and flows as effectively as the channel does.