A tributary, or an affluent, is a stream or river that flows into a larger stream (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 into which they flow, drain the surrounding drainage basin of its surface water and groundwater, leading the water out into an ocean, another river, or into an endorheic basin.
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).
