In its early stage a stream carries water only after a rainfall and is said to be an intermittent stream. In contrast, a permanent stream is one which has cut its valley deeply enough that groundwater seeps into it and keeps it flowing between rainfalls. At first the rivulet may form downslope but in time the rainwater runs together further upslope and gradually works headward until the head is at or near the divide or top of the mountain. This is headward erosion . If the same process occurs on the other side of the divide, erosion occurs in opposite directions, thereby lowering the divide at the meeting point and creating a saddle .
Saddles are fairly common in West Virginia mountain ridges.
If one side of the mouintain is steeper than the other side, the former's stream erodes faster and may cut across the saddle and form a gap or pass . The stream then has its source on one side of the mountain and its headwater on the other, this being a water gap . A prominent example is that of Seneca Creek through Fore Knobs Mountain west of Seneca Rocks. Once the stream cuts through the gap to the level of the valley on the other side, it may erode its way onto the other stream and divert the latter's path. This is stream piracy.