Watershed Information

A watershed is an area of land that drains all water above and below ground into a common stream, river, wetland, lake, or ocean.  Watersheds that are large in scale are called basins. Watersheds that are small in size are called sub-watersheds. Watersheds can cross state, county, and municipal boundaries.

Watersheds are classified by the US Geological Survey (USGS) by Hydrologic Unit Codes (HUCs) which are unique codes with two to eight digits, based on the level of watershed. The larger the HUC number, the smaller the size of the watershed. For example, HUC-02 are the largest watersheds, dividing the nation into 21 regions. HUC-04 divides these regions further into 221 subregions. HUC-06 are divided into 378 hydrologic accounting units. From there, the USGS Watershed Boundary Dataset (WBD) further breaks watersheds into HUC-08, HUC-10, and HUC-12. HUC-14 and HUC-16 are being developed at the moment and are not available for all of the US. All current HUCs are calculated at the 1:250,000 scale. Explore the watersheds of the US with ArcGIS Online. Shapefiles of these watersheds can be downloaded from NRCS Geospatial Data Gateway.

In the Southwestern Pennsylvania region, there are four HUC-04 watersheds: Allegheny, Monongahela, Ohio, and a small portion of the Susquehanna watershed. Find information about each watershed below: