There are three major river basins in Pennsylvania: the Susquehanna, the Ohio, and the Delaware. Together they drain more than 90 percent of Pennsylvania’s land area. Most of eastern and central Pennsylvania is drained by the Susquehanna and Delaware systems. The western part of the state is drained by the Allegheny and Monongahela rivers, which join at Pittsburgh to form the Ohio. In addition to the three major river basins, short streams flowing into Lake Erie drain the northwestern fringe of the state; a small area of Potter County, in north-central Pennsylvania, is drained by the Genesee River into Lake Ontario; and parts of south-central Pennsylvania are drained by tributaries of the Potomac River.
Many of the state’s rivers and streams flow through mountainous or hilly regions, often cutting spectacular gorges and water gaps, which have provided excellent natural passageways for railroads and highways. Numerous dams and reservoirs in the state are designed to control flooding, generate power, support recreation, and provide drinking water.
The Delaware River forms the state’s eastern boundary. The sea has invaded the lower portion of the Delaware and flooded the adjacent coastal plain, creating a tidal estuary. The river has long been important commercially. Oceangoing ships can sail up the Delaware to Philadelphia and go as far north as Trenton, New Jersey. The Susquehanna River rises in two main branches.
The northern portion of the river, called the North Branch, enters northeastern Pennsylvania from New York and follows a winding course southward. The West Branch begins in the Allegheny Mountains and flows generally eastward. The two branches meet at Sunbury in Northumberland County, and continue southward to eventually empty into Chesapeake Bay. The Susquehanna’s principal tributary, the Juniata River, also begins in the Allegheny Mountains and follows a twisting course eastward through a series of mountain ridges before joining the Susquehanna River upstream from Harrisburg.
The Allegheny River rises in the northwest and flows generally southward. The Monongahela River enters the state from West Virginia and flows northward. The junction at Pittsburgh of these two rivers gives birth to the Ohio River, which flows westward to its eventual junction with the Mississippi River. The rivers of southwestern Pennsylvania are important transportation routes, particularly for the transport of coal.
Since northeastern and northwestern Pennsylvania were once glaciated, these regions have many small natural lakes and ponds. Conneaut Lake, which covers 376 hectares (929 acres) in Crawford County, is the state’s largest natural lake. However, several lakes created when dams were built are considerably larger. These include Pymatuning Reservoir, which is located just west of Conneaut Lake on the Pennsylvania-Ohio state line, and Lake Wallenpaupack in the Pocono Mountains. "Pennsylvania" © Emmanuel BUCHOT, Encarta, Wikipedia
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