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One system, developed by [[Arthur Newell Strahler|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:Otto Pfafstetter|Otto Pfafstetter]], known as the [[:pt:Codificação de Bacias Hidrográficas de Otto Pfafstetter|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>
==Europe==
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[[Image:Huc region-edit.png|thumb|right|The 21 top-level 2-digit "region" hydrologic unit boundaries. The names are as follows: 01 New England, 02 Mid-Atlantic, 03 South Atlantic-Gulf, 04 Great Lakes, 05 Ohio, 06 Tennessee, 07 Upper Mississippi, 08 Lower Mississippi, 09 Souris-Red-Rainy, 10 Missouri, 11 Arkansas-White-Red, 12 Texas-Gulf, 13 Rio Grande, 14 Upper Colorado, 15 Lower Colorado, 16 Great Basin, 17 Pacific Northwest, 18 California, 19 Alaska, 20 Hawaii, 21 Caribbean.]]
The regions (1st level hydrologic units) are geographic areas that contain either the [[Drainage basin|drainage area]] of a major river, such as the Missouri region, or the combined drainage areas of a series of rivers, such as the Texas–Gulf region. Each subregion includes the area drained by a river system, a [[Reach (geography)|reach]] of a river and its tributaries in that reach, a [[Endorheic basin|closed basin]] or basins, or a group of streams forming a coastal drainage area.<ref name=HUmaps>{{cite web |title= Hydrologic Unit Maps, U.S. Geological Survey Water-Supply Paper 2294 |last1= Seaber |first1= Paul R. |last2= Kapinos |first2= F. Paul |last3= Knapp |first3= George L. |publisher= [[United States Geological Survey]] |url= http://pubs.usgs.gov/wsp/wsp2294/pdf/wsp_2294.pdf |accessdate= 3 November 2010}}</ref> Regions receive a two-digit code. The following levels are designated by the addition of another two digits.<ref>{{cite web|url=http://www.nrcs.usda.gov/programs/rwa/Watershed_HU_HUC_WatershedApproach_defined_6-18-07.pdf |title=Watersheds, Hydrologic Units, Hydrologic Unit Codes, Watershed Approach, and Rapid Watershed Assessments |publisher=[[USDA]] |accessdate=2010-10-27 |deadurl=yes |archiveurl=https://web.archive.org/web/20101012233329/http://www.nrcs.usda.gov/programs/rwa/Watershed_HU_HUC_WatershedApproach_defined_6-18-07.pdf |archivedate=2010-10-12 |df= }}</ref> The hierarchy was designed and the units subdivided so that almost all the subbasins (formerly called cataloging units) are larger than {{convert|700|sqmi|km2}}. Larger closed basins were subdivided until their subunits were less than 700 square miles.<ref name=HUmaps/> The 10-digit watersheds were delineated to be between 40,000 and 250,000 acres in size, and the 12-digit subwatersheds between 10,000 and 40,000 acres.<ref name=WBDstandard/> In addition to the hydrologic unit codes, each hydrologic unit was assigned a name corresponding to the unit's principal hydrologic feature or to a cultural or political feature within the unit.<ref name=HUmaps/>
The boundaries of the hydrologic units usually correspond to drainage basins with some exceptions; for example, subregion 1711, called "Puget Sound", includes all U.S. drainage into not only Puget Sound but also the [[Strait of Georgia]], [[Strait of Juan de Fuca]], and the [[Fraser River]].<ref>{{cite web |url= http://wa.water.usgs.gov/data/wuse/huc.names.txt |title= List Hydrologic Unit Codes (HUCs) - USGS Washington |publisher= [[USGS]] |accessdate= 19 July 2011}}</ref> Also, region and subregion boundaries end at the U.S. international boundary.<ref name=HUmaps/>
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