One system, developed by [[Arthur Newell Strahler]], known as the [[Strahler number|Strahler stream order]], ranks streams based on a hierarchy of tributaries. Each segment of a stream or river within a river network is treated as a node in a tree, with the next segment downstream as its parent. When two first-order streams come together, they form a second-order stream. When two second-order streams come together, they form a third-order stream, and so on.
Another example is the system of assigning IDs to watersheds devised by [[:pt:{{ill|Otto Pfafstetter|Otto Pfafstetter]]pt}}, known as the [[Pfafstetter Coding System]] or the Pfafstetter System. Drainage areas are delineated in a hierarchical fashion, with "level 1" watersheds at continental scales, subdivided into smaller level 2 watersheds, which are divided into level 3 watersheds, and so on. Each watershed is assigned a unique number, called a Pfafsetter Code, based on its ___location within the overall drainage system.<ref>[http://proceedings.esri.com/library/userconf/proc01/professional/papers/pap1008/p1008.htm Watershed Topology - The Pfafstetter System] {{webarchive|url=https://web.archive.org/web/20110710202642/http://proceedings.esri.com/library/userconf/proc01/professional/papers/pap1008/p1008.htm |date=2011-07-10 }}, by Jordan Furnans and Francisco Olivera</ref>
