Some software systems and tools use watersheds as their unit of observation. This unit of analysis more or less naturally lends itself to ecologically oriented analyses, but socioeconomic data can certainly be attributed to such features. The most widely used system in the US for delineating watershed boundaries is the hydrologic unit classification (HUC) system of the US Geological Survey. The USGS HUC system starts by delineating basins (e.g., the Columbia River Basin), which are then hierarchically divided into subbasins and so on. The term watershed technically refers to the 5th level of the HUC system, with the 6th level representing subwatersheds. However, the term watershed is often used more loosely to refer to HUC units of the 5th level and lower, in which case it is helpful to qualify the meaning of watershed by including the HUC level (e.g., 6th-code watershed).
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