Fall Line

In geomorphology, a fall line marks the area where an upland region (continental bedrock) and a coastal plain (coastal alluvia) meet. Technically, a fall line is an unconformity. A fall line is typically prominent when crossed by a river, for there will often be rapids or waterfalls. Because of these features river boats typically cannot travel any farther inland without portaging unless locks are built. Because of the need for a port and a ready supply of water power, settlements often develop where rivers cross a fall line.
See more at Wikipedia.org...