Ice Age and History of Surface Water in SW PA.

Ice age

Beringia

Siberia and (current) Alaska

North America during Ice Age

The History of Surface Water

Because this region (and almost all of western Pennsylvania) is part of the Ohio River drainage system, it is important to understand some of the basic history of The Ohio. The precipitation that falls to the surface, about 36 " a year, makes its way to the Ohio River through a well-developed network of tributaries, and ultimately, the waterway ends up in the Mississippi and then the Gulf of Mexico. This, however, was not always the case.

Near the start of the current era, the Cenozoic, this region was most likely a low plain. It had  meandering streams flowing near base level and eroding laterally (in roughly the same channels as they are in now). Uplift occurred ( or sea level dropped), rejuvenating the streams, which began down cutting their channels 300-400ft. Over time, vertical erosion slowed and lateral cutting dominated.

One of the most interesting aspects of Pennsylvania rivers is that no major system empties into nearby Lake Erie. Even more interesting is why.

Erie Basin

Evidence suggests that past drainage from the area trended north along the ancestral Monongahela River into the St. Lawrence Basin. 

As it does now, the Monongahela flowed north to the area now known as Pittsburgh. It then continued northward into Beaver and emptied into the Erie Basin. During this time, the ancestral Ohio River was  a tributary to the  Monongahela. Using topographical maps or digital elevation models, like those below, you can trace the ancient Monongahela's channel. Locally, the past locations of the Monongahela, Allegheny, and  the Ohio stream channels present themselves as high terraces, referred to as the Parker Strath in Pittsburgh.

Current and Past River Channels

The advance of the Illinoian Glaciers had a tremendous effect on this regions streams. The advancing ice acted as a natural dam and blocked the northerly flowing Monongahela River, creating a large lake. The lake became so deep that the river reversed its course and has flowed south ever since. 

Terraces

The newly formed Ohio flowed along much of the southernmost edge of the glacier to Cincinnati, thus highlighting its extent. The retreating ice cause an uplift event and the areas sediment laden stream cut into their channel, leaving behind terraces which now serve to mark their former locations.

This change of direction sent the newly formed Ohio river south where it connects to the Big Muddy (Mississippi River). This makes it possible to ship materials from Canada (once New France), all the way to Louisiana, using mainly a boat. The gave it great strategic value, especially to the French in commerce and in War. Whatever European nation controlled The Ohio River  would ultimately control the new world.

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