FIGURE 2-2 The Allegheny and Monongahela watershed. SOURCE: USGS, 1995.

The Pittsburgh region lies primarily in the Appalachian Plateau, which extends southward from New York to Alabama (see Figure 2-3) and is shaped by a geologic history that reflects the bituminous coal fields that have served as an important economic driver for the region for more than a century. However, historical and ongoing extraction of fossil fuels has left the region with a legacy of coal refuse piles, stripped landscapes, and acid mine drainage.

The physical landscape of the region was shaped in part by glaciation. As glaciers moved south during a series of ice ages, they reversed the course of the ancient Monongahela and Allegheny Rivers, which at one time flowed northward into the ancestral Lake Erie basin. As noted by John Harper (1997) of the Pennsylvania Bureau of Topographic and Geologic Survey: