What is the area called where a river's flow is influenced by its tributaries?

The area where a river's flow is influenced by its tributaries is known as a drainage basin. This term refers to the region or area of land where all precipitation collects and drains into a common outlet, such as a river or lake. The tributaries are smaller streams or rivers that feed into the main river, contributing to its flow and volume. Understanding the drainage basin is crucial as it helps in analyzing how water moves through the landscape, how it collects from various sources, and how human activities can impact this hydrological process.

The other terms relate to different concepts: the source is where the river begins, the mouth is where it flows into a larger body of water, and the watershed refers to the dividing line that separates different drainage basins. Each term has its specific meaning and relevance in the study of river systems.