Tributary

A tributary, or an affluent, is a stream that typically flows into a larger stream (main stem or parent) or lake, or rarely into a smaller stream, e.g., the Esla tributary to the Douro. A tributary does not flow directly into a sea or ocean. Tributaries and the main stem into which they flow drain the surrounding drainage basin of its surface water and groundwater, leading the water out into an ocean, another stream, or into an endorheic basin.

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. Distributaries are most often found in river deltas.

The Irtysh, a tributary of the Ob river, is 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 m/s (1.1 million cu ft/s).