Which term best describes the combined area impacted by both a river and its tributaries?

The term that best describes the combined area impacted by both a river and its tributaries is the drainage basin. A drainage basin, also known as a catchment area, encompasses all the land where precipitation collects and drains into a specific river system, including its tributaries. This area is crucial for understanding how water flow, sediment transport, and ecosystems are interrelated within that geographical boundary.

A watershed, while closely related, typically refers to the high land separating different drainage basins. It doesn't capture the full extent of both the main river and its tributaries as a drainage basin does. The mouth of a river is where it meets a larger body of water, and the source is the point from which the river originates. Neither of these terms represents the complete collection area of water contributing to the river system. Thus, the drainage basin is the most accurate and encompassing term in this context.