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A 'tributary' (or 'confluent'/'affluent') is a stream or river which flows into a mainstem (or parent) river, and which does not flow directly into a 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 means 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.
The Strahler Stream Order examines the arrangement of tributaries in a hierarchy of first, second, third and higher orders, with the first order tributary being typically the least in size. For example, a second order tributary is comprised of two or more first order tributaries combining to form the second order stream.
Exampe rivers flowing into Missisoi river