The [[United States Geological Survey]] created a hierarchical system of hydrologic units originally called regions, sub-regions, accounting units, and cataloging units. Each unit was assigned a unique '''Hydrologic Unit Code''' (HUC). As first implemented the system had 21 regions, 221 subregions, 378 accounting units, and 2,264 cataloging units.<ref name="USGS HUCs">{{cite journal|last=Seaber, Paul R., F. Paul Kapanos, and George L. Knapp|title=Hydrologic Unit Maps|journal=United States Geological Survey Water-supply Papers|year=1987|volume=No. 2294|pages=i-iii, 1-631–63|accessdate=28 September 2011}}</ref><ref name=usgs>{{cite web|title=Hydrologic Unit Maps - What are Hydrologic Units?|url=http://water.usgs.gov/GIS/huc.html|publisher=[[USGS]]|accessdate=2010-10-27}}</ref> Over time the system was changed and expanded.<ref>{{cite web |title= Overview and History of Hydrologic Units and the Watershed Boundary Dataset (WBD) |publisher= [[Natural Resources Conservation Service]] |url= http://www.ncgc.nrcs.usda.gov/products/datasets/watershed/history.html}}</ref> As of 2010 there are six levels in the hierarchy, represented by hydrologic unit codes from 2 to 12 digits long, called regions, subregions, basins, subbasins, watersheds, and subwatersheds. The table below describes the system's hydrologic unit levels and their characteristics, along with example names and codes.<ref>{{cite web |title= Watershed Boundary Dataset (WBD) Facts |publisher= [[Natural Resources Conservation Service]] |url= http://www.ncgc.nrcs.usda.gov/products/datasets/watershed/facts.html}}</ref>
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