- National Watershed Boundary Dataset (NWBD): The NWBD, developed by the United States Geological Survey (USGS), divides the United States into 2,110 hydrologic units (HUs) that are categorized into four levels: regions, subregions, accounting units, and cataloging units. Each HU can be considered a watershed, so the NWBD suggests there are approximately 2,110 major watersheds in the United States.
- Hydrologic Unit Code (HUC): The HUC system is also developed by the USGS and further subdivides the NWBD HUs into smaller watersheds. There are a total of 21 HUC regions, 222 HUC subregions, 352 HUC accounting units, and 2,268 HUC cataloging units. Each of these HUC units represents a watershed, so there are approximately 2,268 HUC watersheds in the United States.
- Smaller Watersheds: Within the larger watersheds defined by the NWBD and HUC systems, there are numerous smaller watersheds that may not be officially designated or named. These smaller watersheds can range from a few acres to several square miles and may be defined based on local topography, land use, or drainage patterns. The exact number of these smaller watersheds is difficult to determine and can vary depending on the criteria used to define them.
Therefore, the number of watersheds in the United States depends on the scale and definition used. The NWBD suggests there are approximately 2,110 major watersheds, while the HUC system further divides the country into 2,268 HUC watersheds. Additionally, there are numerous smaller watersheds that are not officially designated or named.
