Tributary

For other uses, see Tributary (disambiguation)}}}.

A tributary (or confluent) is a secondary or subsidiary stream or river which flows into another river (a parent river) or body of water but which may not flow directly into the sea. In orography, tributaries are ordered from those nearest to the source of the river to those nearest to the mouth of the river. A confluence is where two or more tributaries or rivers flow together.

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.

Relativity of the notion

In most cases, at a confluence it is clear which stream is the main river and which is the tributary, as one stream is both much longer and carrying clearly more water than the other.

It can, however, happen that one stream is longer, but the other carries more water. Which stream is then the main river, and which is the tributary?

There are no fixed rules for answering this question. In Switzerland, for instance, the Alpine Rhine, which is longer than the Aare, is chosen as the main stream, although the Aare carries more water. In the case of upper-Mississippi (which carries more water) and Missouri (which is much longer) the choice is made according to the opposite criterion.

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