Tributary
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.
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.
