For what it's worth, here's my two cents on the subject. I don't know if it's geologically correct, but it is how I grew up understanding it. Helen, I am assuming that you are familiar with the shape of the Appalachian mountain range? It is a fold range, therefore all the mountains are very long and relatively narrow and run in a SW to NE direction. The low places between each 'fold', running parallel to the tops of the mountains are the valleys, but if you stand in a valley and look at the face of the mountains, you see little valleys that are perpendicular to the mountain top cut into the face. These are hollows. In many places, if you walk into these hollows, you can find traces of old farms because this is where many of the old pioneers chose to settle. Why? My guess would be because they were easily defined with definate borders, had water (most of them will have a small stream) and were far enough away from the main creek to keep them out of the floods, but still close enough to provide navigation. Many of these hollows still have the name of the first settler. River (Susquehanna), Branch (East, West, Bennetts, etc), Creek (West, Clear, Pine, etc), Run are all names of waterways, and usually named in size order. Going upstream, the rivers break into branches, the branches into Creeks, and creeks are built by the combination of Runs. Of course, that's just a general rule, because some of the runs can empty straight into the river, but you get the general idea.