The topography of the land has a major role in the direction of water flow within a watershed. As rainwater moves from mountains and ridges, through valleys and creeks into streams and rivers, it follows the path of least resistance.
But why does some of it soak into the ground while more of it just continues downstream. And how does the water underground effect what the path the surface water takes? These questions can be answered by taking a history lesson in the Geological Story of PA.