Pittsburgh, Pennsylvania is situated on the confluence of the Allegheny and Monongahela Rivers to form the Ohio.
In A Long Walk to Pittsburgh: Part 1, Topanga Lawrence reveals that she is moving to Pittsburgh with her family. Despite being separated by 300 miles, she and Cory Matthews decide to stay together. She returns to Philadelphia in the next episode to be with Cory and stays with her aunt Prudence for the remainder of high school.
At the beginning of Season 7, Cory travels to Pittsburgh to attempt to reunite Topanga's parents, Jedidiah and Rhiannon so he and Topanga can be together. He fails miserably, but her parents talk her around, and they get married a few episodes later.