A tributary or affluent is a stream or river that flows into a main stem (or parent) river or a lake. A tributary does not flow directly into a sea or ocean. Tributaries and the mainstem river serve to drain the surrounding drainage basin of its surface water and groundwater by leading the water out into an ocean or sea.

A confluence, where two or more bodies of water meet together, usually refers to the joining of tributaries.

Reciprocal to a tributary is a distributary, a river or stream that branches off from and flows away from the main stream. Distributaries are most often found in river deltas.

'Right tributary' and 'left tributary' (or 'right-bank tributary' and 'left-bank tributary') are terms stating the orientation of the tributary relative to the flow of the mainstem river. These terms are defined from the perspective of looking downstream (in the direction the water current of the main stem is going).

Where tributaries have the same name as the river into which they feed, they are called forks. These are typically designated by compass direction. For example, the American River receives flow from its north, middle, and south forks.

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