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[1] to the topological ordering of rivers or sections of rivers based on their distance from the source ("top down"[2]) or from the confluence (the point where two rivers merge) or river mouth ("bottom up"[3]), and their hierarchical position within the river system. As terminology, the words "stream" and "branch" tend to be used rather than "river".