The stream order or waterbody order is a positive whole number used in geomorphology and hydrology to indicate the level of branching in a river system.
There are various approaches to the topological ordering of rivers or sections of rivers based on their distance from the source ("top down") or from the confluence (the point where two rivers merge) or river mouth ("bottom up"), and their hierarchical position within the river system. In terms of terminology, the words "stream" or "branch" tend to be used rather than "river".
The classic stream order, also called Hack's stream order or Gravelius' stream order, is a "bottom up" hierarchy that allocates the number "1" to the river with its mouth at the sea (the main stem). Its tributaries are given a number one greater than that of the river or stream into which they discharge. So, for example, all immediate tributaries of the main stem are given the number "2". Tributaries emptying into a "2" are given the number "3" and so on.
This stream order starting at the mouth indicates the river's place in the network. It is suitable for general cartographic purposes, but can pose problems because, at each confluence, a decision has to be made about which of the two branches is a continuation of the main channel, or whether the main channel has its source at the confluence of two other smaller streams. The first order stream is the one which, at each confluence, is the one with the greatest volumetric flow, which usually reflects the long-standing naming of rivers. Associated with this stream order system was the quest by geographers of the 19th century to find the "true" source of a river. In the course of this work, other criteria were discussed to enable the main stream to be defined. In addition to the stream with the greatest lengith (the source at the maximum distance from the mouth) and the size of the various catchments, account was taken of the stream which deviated least at the actual confluence as well as the mere successive names of rivers such as the Rhine and the Aare or the Elbe and the Vltava.
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