Stream Formation and Erosion            Downhill. Water running down a slope becomes a stream when there is enough water to form a tiny rivulet with a channel to contain the water. 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. 

      Gaps. 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. To view Saddleback Mountain, Seneca Rocks, and Blackwater Falls:  http://www.ovis.net/~rspencer/spring5.htm

     A low gap is the present river path and a high gap is the older stream course or other erosion gap. The Blue Ridge mountains in Virginia have many high gaps.

     Gaps were the land-gateways to what is now West Virginia. Pioneers leaving the Great Valley (where I-81 is today) moved through the mountains at a few gaps. [See Mountains]. Travel in colonial America primarily was on or beside waterways. Development of counties or other units naturally followed to protect these water routes.

      Valleys. All streams, regardless of size, occupy valleys.  The course of the stream at the lowest point of the valley is its channel.   Moving water picks up weathered material and cuts through both soil and bedrock and deepens the valley. A young stream creates steep-sided, V-shaped valleys such as the Blackwater Canyon.  If the stream's gradient is steep, waters may plunge over rapids or waterfalls, Blackwater Falls being an excellent example.  Usually this occurs where the stream crosses rock layers harder than downstream rocks. In time as the stream's gradient lessens,  the stream matures and eliminates waterfalls and rapids and meanders. 

     Through centrifugal force the curves of the stream widen.  As the stream gets old, the valley bottom becomes broad as the stream meanders in great curves. The lowest level to which a stream may erode its valley is base level.  Sea level is the base level for rivers flowing into the sea. A tributary can reach the base level of the main stream.  Every major stream controls the base level of the streams flowing into it. 
Last updated on Tuesday, July 25, 2000