What Is a Watershed?
“Watershed” is a generic term used to identify an area of land that drains to a particular waterbody. Watersheds can vary in size, from the acreage that drains into a brook to a major river. For purposes of this atlas, watersheds are classified by a nested hierarchy based on landscape scale. A watershed is the land area that drains into a stream or river (or in some cases, two streams) and is the smallest in size in the classification hierarchy. Pennsylvania’s original State Water Plan divided the commonwealth into 104 watersheds, ranging in size from approximately 100 to 1,000 square miles, named for the major streams of the watershed. A subbasin includes all of the watersheds that drain into a particular reach of a larger watercourse. A basin encompasses all of the subbasins into a major waterway. In Pennsylvania there are six basins—Erie, Genesee, Ohio, Susquehanna, Potomac and Delaware—each with a different outlet. The Erie basin empties into Lake Erie, the Genessee basin contributes to Lake Ontario, the Ohio basin drains into the Mississippi River, the Susquehanna Basin and the Potomac Basin drain into the Chesapeake Bay, and the Delaware Basin drains into the Delaware Bay.
Pennsylvania watershed basins. Image source: Allegheny Watershed Alliance
A particular tract of land can contain multiple watersheds, depending on the scale of the landscape. For example, several small streams drain into Spring Creek, which is a tributary to the West Branch of the Susquehanna River, which is a tributary to the Susquehanna River. The Pennsylvania Department of Environmental Protection divides the State into six major watershed regions: the Delaware Region, the Potomac Region, the Ohio Region, the Great Lakes Region, the Lower Susquehanna Region and the Middle/Upper Susquehanna Region which is where the Spring Creek Watershed is located.