Pennsylvania can be divided into more than 12 different geographic regions. In the southeastern part of the state is a section of the Atlantic Coastal Plain, a narrow strip of sandy low-lying land along the Delaware River. This region was the site of William Penn’s settlement. Inland from the Coastal Plain is the Piedmont Plateau. This is a gently rolling plain that is less than 500 feet (150 meters) above sea level. The eastern part of this plain is called the Piedmont Upland. The boundary between the Piedmont and the Coastal Plain is known as the fall line. Parallel to the Piedmont Plateau is the Piedmont Lowland. The limestone rocks of this area have weathered to create fertile lowlands, such as the Conestoga Lowlands of Lancaster county.
Farther to the northwest is the northernmost part of the Blue Ridge Mountains. Inland from the Blue Ridge is the Ridge and Valley Province. This is made up of long, narrow valleys and ridges. If you could see it from space, this area would look as if an enormous rake had been dragged along the Appalachians from northeast to southwest. To the east is the Great Valley, which stretches more than 1,200 miles (1,930 km) from Pennsylvania to Alabama. The Lehigh is part of the Great Valley. To the west and north of the Great Valley is the Pocono Plateau, which rises to about 2,200 feet (700 meters).
Most of the way from the north-central part of the eastern border to the west-central part of the southern border are the Appalachian Mountains. The Allegheny High Plateau, makes up the western and northern parts of the state. The Allegheny Front, an escarpment along the eastern edge of the plateau, is the most striking geographical feature in Pennsylvania. It is broken up by many winding streams which form narrow, steep-sided valleys. The southwestern part of the Allegheny High Plateau the highest point in the state at 3,213 feet (979 meters). A narrow lowland region, the Erie Plain, borders Lake Erie in the extreme northwestern part of the state.
Pennsylvania has three major river systems. In the east is the Delaware River, fed mainly by the Lehigh and Schuylkill rivers. The Delaware forms Pennsylvania’s eastern border and flows to the Atlantic Ocean. In the central part of the state is the Susquehanna, a wide, shallow stream that meanders into Maryland and Chesapeake Bay. In the west is the Ohio River—formed by the meeting of the Allegheny (north) and Monongahela (south) rivers at Pittsburgh—from where it flows westward to the Mississippi River. The Beaver, Clarion, and Youghiogheny rivers are also important rivers.
Pennsylvania also has more than 735 sq mi (1,904 sq km) of Lake Erie, and about 250 natural lakes larger than 20 acres (8 ha). These lakes are mostly in the northeast and northwest. The largest natural lake is Conneaut Lake, about 30 mi (48 km) south of the city of Erie. The largest artificial lake is Lake Wallenpaupack, in the Poconos.
Pennsylvania has a humid continental climate, with big differences in the temperatures in summer and winter. The regions with the warmest temperatures and the longest growing seasons are the low-lying southwest Ohio valley and the Monongahela valley in the southeast. The region bordering Lake Erie also has a long growing season, as the moderating effect of the lake prevents early spring and late autumn frosts. The first two areas have hot summers, while the Erie area is more moderate. The rest of the state, at higher elevations, has cold winters and cool summers.
The average temperature in July is about 70 °F (21 °C) and in January about 28 °F (− 2 °C), although the temperatures can change a great deal from one region to the next. Around 40 inches (1,000 mm) of precipitation fall in the state annually. The record low temperature for the state is –42°F (–41°C), set at Smethport on 5 January 1904; the record high, 111°F (44°C), was reached at Phoenixville on 10 July 1936.
There have been several large floods in the state’s history. On 31 May 1889, the South Fork Dam near Johnstown broke after a heavy rainfall, and its rampaging waters killed 2,200 people and destroyed the entire city in less than 10 minutes. On 19–20 July 1977, Johnstown experienced another severe flood, resulting in 68 deaths. Rains from Hurricane Agnes in June 1972 resulted in floods that caused more than $1.2 billion worth of damage in the Susquehanna Valley.
