13.3 River Valleys
Headward Erosion: is a process of erosion that
lengthens a stream, a valley or a gully at its
beginning and also enlarges its drainage basin.
Canyon & V-Shaped Valley Evolution
1.
2.
V-Shaped Valley Starts:
1.
V-Shaped Valley Starts:
2.
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Stream Piracy – The
Development of a Watergap
Before
After
Stream capture, or stream piracy, is a geomorphic
phenomenon that occurs when a stream or river from a
neighboring drainage system or watershed erodes
through the divide between two streams and "captures"
another stream which then is diverted from its former
bed and now flows down the bed of the capturing
stream. This can occur through headward erosion of one
stream valley into another or through lateral erosion of
meanders through the sediments between parallel
streams.
An example!
Stage 1 Beaverdam
Creek, Gap
Run, and
Goose Creek
flow eastward
through the
Blue Ridge
and enter the
Potomac.
(from Judson and Kauffman)
Stage 2 - As the land
is eroded downward,
the three east flowing
creeks do not have the
power to erode as far
through the Blue Ridge
as the Shenandoah,
Potomac system. The
Shenandoah extends
itself southward by
headward erosion
through the relatively
high land west of the
Blue Ridge. It
eventually captures
Beaverdam Creek.
An example!
(from Judson and Kauffman)
An example!
Stage 3 - The capture
of Beaverdam Creek
added more discharge
to the Shenandoah
which was able to
therefore erode more.
Headward erosion
leads to the capture of
Gap Run. The water
gaps where Beaverdam
Creek and Gap Run
used to flow through
the Blue Ridge are left
as wind gaps.
(from Judson and Kauffman)
An example!
Stage 4 - Eventually
Goose Creek is
captured as well.
Snicker's Gap, Ashby
Gap, and Manassas
Gap are left as wind
gaps. As the land on
either side of the ridge
is eroded down
together with the ridge
summit, the relative
elevation of the wind
gaps becomes higher
and higher.
(from Judson and Kauffman)
Summarizing
Erosion and downcutting
through young
horizontal units
Folded Units
Development of
water gap
Waterfall Formation
A waterfall is usually
a geological formation
resulting from water,
often in the form of a
stream, flowing over
an erosion-resistant
rock formation that
forms a sudden break
in elevation.
Some waterfalls form in mountain environments where
erosion is rapid and stream courses may be subject to
sudden and catastrophic change. In such cases, the
waterfall may not be the end product of many years of
water action over a region, but rather the result of
relatively sudden geological processes such as
landslides, faults or volcanic action.
Niagara Falls –
Water action
erodes weak
shales at the
base of the falls.
This erosion
undermines the
tough dolostone
layer above.
From time to
time, the
dolostone
breaks off, and
the waterfall
recedes.