Tributary

A tributary,^[1] or an affluent,^[2] is a stream or river that flows into a larger stream (main stem or "parent"), river, or a lake.^[3] A tributary does not flow directly into a sea or ocean.^[4] Tributaries, and the main stem river into which they flow, drain the surrounding drainage basin of its surface water and groundwater, leading the water out into an ocean.

The Irtysh is a chief tributary of the Ob river and is also the longest tributary river in the world with a length of 4,248 km (2,640 mi). The Madeira River is the largest tributary river by volume in the world with an average discharge of 31,200 m³/s (1.1 million cu ft/s).

A confluence, where two or more bodies of water meet, 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.^[5] Distributaries are most often found in river deltas.