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 main stem 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.

The opposite 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.