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. Network analysis examines the arrangement of tributaries in a hierarchy of 1st, 2nd, 3rd etc. orders.
A tributary joins another river at a confluence.
The opposite of a tributary is a distributary; a river branch that flows away from the main stream.