A tributary or affluent[1] is a stream or river that flows into a main stem (or parent) river or a lake.[2] A tributary does not flow directly into a sea or ocean. Tributaries and the mainstem river serve to drain the surrounding drainage basin of its surface water and groundwater by leading the water out into an ocean or sea.
A confluence, where two or more bodies of water meet together, usually refers to the joining of tributaries.
Reciprocal to a tributary is a distributary, a river that branches off of and flows away from the main stream. Distributaries are most often found in river deltas.
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'Right tributary' and 'left tributary' (or 'right-bank tributary' and 'left-bank tributary') are terms stating the position of the tributary relative to the flow of the mainstem river. These terms are defined from the perspective of looking downstream (in the direction the water current is going).
Where tributaries have the same name as the river into which they feed, they are called forks. These are typically designated by compass direction. For example, the American River receives flow from its north, middle, and south forks.
Forks are sometimes designated as right or left. Here, the "handedness" is relative to an observer facing upstream. For instance, Steer Creek has a left tributary which is called Right Fork Steer Creek.
In geography, tributaries are sometimes ordered from those nearest to the source of the river to those nearest to the mouth of the river. For example, 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 would be composed of two or more first order tributaries combining to form the second order tributary.
Another method is to organize tributaries from mouth to source, in the form of a tree structure, stored as a tree data structure.