What Is A Watershed?
A watershed is an area of land where rain or snow melt flows downhill to a body of surface water such as a stream, pond, lake, or river. Alternative terms for such an area include drainage basin, catchment, catchment basin, or drainage area. The US Environmental Protection Agency definition is "Watershed - An area of land that drains water, sediment and dissolved materials to a common receiving body or outlet. The term is not restricted to surface water runoff and includes interactions with subsurface water. Watersheds vary from the largest river basins to just acres or less in size. In urban watershed management, a watershed is seen as all the land which contributes runoff to a particular water body." The watershed, or basin, includes all the streams uphill from the point of definition as well as the landscapes through which those stream flow.
Watershed Diagram Courtesy of Arkansas Watershed Advisory Groups, prairierivers.org
Watersheds exist on many scales with smaller drainages flowing into ever larger streams. Small subwatersheds might drain only a few acres on a single slope of a hillside or mountain while river basin watersheds collect rain and meltwater from mant thousands of square miles. In Pennsylvnia, water flowing across our lands travels by six different routes to reach the ocean. The Potomac and Susquehanna River basins empty to the Chesapeake Bay before entering the Atlantic Ocean while the Delaware River enters Delaware Bay, another arm or the Atlantic. Along our northern border, the small darinage area of the Genesee River flows borth to Lake Ontario and the streams along the Lake Erie coastline feed water to that lake. These waters join with those from all the upper Great Lakes , flowing to the Atlantic Ocean by way of the St. Lawrence River. And, the waters of the western third of Pennsylvania, drained by rivers such as the Conemaugh, Monongahela, Youghiogheny, and Allegheny, are carried to the Gulf of Mexico by the Ohio and Mississippi Rivers. The illustration below depicts these six major basins in Pennsylvania.
The map below illustrates the land area of the six states and the District of Columbia that comprises the Chesapeake Bay and the two major river basins that form the major portion of the watershed within Pennsylvania: the Susquehanna and Potomac basins. Click here to link to the Chesapeake Bay Program map page with many maps illuminating Bay characteristics.
Below is a map of the Susquehanna River basin and the larger sub-basins, including the Juniata River sub-basin shown in grey. The Susquehanna River watershed covers 27,500 square miles in Maryland, Pennsylvania, and New York. The Juniata River sub-basin is 3400 square miles in area. The Raystown Branch of the Juniata, the largest Juniata tributary, is 118 miles long and drains an area of 962 square miles, including most of the northern two-thirds of Bedford County. Click here for a pdf version of a Susquehanna River Basin Commission map of the Juniata River watersheds including the Raystown Branch.
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