HUC (hydrologic unit code)
The USGS numbering system that nests watersheds from large regions (HUC2) down to subbasins (HUC8) and smaller.
HUC codes give every watershed in the country a stable identifier, nested so that each smaller unit sits inside a larger one. HUC2 divides the country into 21 major regions; HUC8 subdivides those into roughly 2,200 subbasins.
Because agencies publish against HUCs, they are the only reliable unit for comparing water conditions across state lines. Analysis done by county tells you about politics; analysis done by HUC tells you about water.
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