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A tributary (or affluent or confluent) is a contributory stream, a river that does not reach the sea, but joins another major river (a parent river), to which it contributes its waters, swelling its discharge. A tributary joins another river at a confluence. When a river's tributaries are listed in orographic sequence, they are in order from the highest (nearest the source of the river) to the lowest (nearest the mouth).

The descriptive terms right tributary and left tributary always apply from the perspective of looking downstream (in the direction the current is going), similarly to the river banks.

The opposite of a tributary is a distributary; a river branch that flows away from the main stream.

A river and all its tributaries drain the watershed of the river.

Network analysis examines the arrangement of tributaries in a hierarchy of 1st, 2nd, 3rd etc. orders.