Source: Kim Roberts, Center for Watershed Protection

A watershed is a land area that channels rainfall and snowmelt to creeks, streams, and rivers, and eventually to collection points such as reservoirs, bays, and the ocean. Also referred to as a drainage basin or catchment, a watershed can be described on many scales depending on the collection point being discussed. The watershed for a drinking water reservoir would include all the land that contributes overland flow by streams and simple surface flow to the reservoir and can have a significant affect on water quality if that land is not managed well. The drainage area for Shawnee Lake in Bedford County is about 36.5 square miles, for Raystown Lake the watershed is 958 square miles, for the Chesapeake Bay the watershed is over 64,000 square miles, and the Mississippi River delivers to the Gulf of Mexico water collected from a watershed of 1.15 million square miles, stretching from the Rockies to the Appalachians.

Watersheds of the Commonwealth

Pennsylvania's Major River Basins (map)

Pennsylvania's Six Ways to the Sea (map)

Pennsylvania's total area is 46,055 square miles with 44,816 square miles of land surface and 1,239 square miles of water surface. Flowing water that leaves Pennsylvania eventually enters either the Atlantic Ocean or the Gulf of Mexico. The route to the Gulf of Mexico is by way of the Ohio River that flows west from the confluence at the Point in Pittsburgh and joins the Mississippi River at Cairo, Illinois. The confluence at the point is the juncture of the Allegheny and Monongahela Rivers that together drain much of western Pennsylvania as well as portions of New York, Maryland, and West Virginia.

Pennsylvania waters join the Atlantic by three major routes. In Erie County. a number of small streams flow north into Lake Erie and these waters flow north to tumble over the falls at Niagara and into Lake Ontario where they mingle with some other Commonwealth waters that have travelled north from near Ulysses in Potter County by way of the Genesee River to Rochester, New York. All these waters flow northeast from Lake Ontario toward the Gulf of St Lawrence, via the St.Lawrence River, and into the Atlantic.

Pennsylvania's Lake Erie Coast

From the southern border of Pennsylvania, water flows to the Chesapeake Bay by way of a number of creeks that drain Somerset, Bedford, Fulton, Franklin and Adams Counties as headwaters to the Potomac River. Of course most of the waters entering the Chesapeake from Pennsylvania flow down the mighty Susquehanna River but there are also three small drainages that leave PA but enter the Bay south of the mouth of the Susquehanna at Havre de Grace, Maryland. These are the Gunpowder Falls River, which drains a small portion of York County, and the Elk and Northeast Rivers, which drain parts of Chester County.

Map of Susquehanna River Basin

The third route to the Atlantic is by way of the Delaware River that constitutes the entire eastern border of the Commonwealth, flowing into Delaware Bay which joins the Atlantic between Cape May, New Jersey and Cape Henlopen, Delaware.