Stream Formation and
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:
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
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