What do we call a small river or stream that flows into a larger river?

A small river or stream that flows into a larger river is referred to as a tributary. This term is used in hydrology to describe a smaller body of water that feeds into a larger one, contributing to its flow. Tributaries play a significant role in the drainage system of a river basin, as they help to increase the volume of water in the main river and can also influence its path and flow characteristics.

Understanding this concept is essential for studying river systems and ecosystems. Other terms related to rivers, like 'confluence,' describe the point where two rivers meet, but do not specifically refer to the smaller river itself. Similarly, the 'mouth' refers to the part of the river where it flows into a larger body of water, like an ocean or a lake, while 'watershed' describes the area of land that drains into a particular river or body of water, rather than the river itself. Thus, the term tributary precisely identifies the smaller stream that contributes to a larger river.