Mark William Landry - 2002
Water is one of the most powerful resources in the
world. In places where it is scarce, wars are fought to possess
what 
little
is discovered. In areas where water is found in abundance, severe
flooding can occur. The Rivière des Ha!Ha! of Québec
(right)
and the Red River of Manitoba has shown the world the power of water and
its destructive potential. No human structure could withstand the
most basic of molecules when they decide to combine their forces.
Likewise, the Earth's surface surrenders its own feathers and scales to
this superior invasion. In particular, fluvial erosion is
a weapon of destruction, aiding water to its eventual victory.
Source of photo: http://sts.gsc.nrcan.gc.ca/page1/geoh/saguenay/img68.htm
What is fluvial erosion?
According to Strahler and Strahler (1997), fluvial processes are processes in which running water is the dominant fluid agent, acting as overland flow and stream flow (1). In order for there to be a flow, there must be gravity to keep the water moving. That being the case, all flows must be moving down a slope. Overland flow, or runoff, is the motion of water over a sloping surface (2). If this is continuous, the runoff would erode its pathway and create a stream flow.
Stream flow could be meausured by the discharge,
which is the volume of water flowing through a given area at a given time.
Hydrologists use this equation to calculate discharge:
Q = (d)(v)(w)
The Q represents discharge which is measured in m³/s.
The d represents the depth of the stream which is measured in
metres.
The v represents the velocity of the stream which is measured
in m/s.
The w represents the width of the stream which is also measured
in metres.
If one of the variables measuring discharge increases or decreases, one or both of the other variables must also increase or decrease in order for the discharge to remain the same. For example, if the width of a stream were to suddenly decrease, the velocity of the stream would likley increase to compensate.
Erosion is the combination of processes in
which the materials of the earth's surface are loosened, dissolved, or
worn away, and transported from one place to another by natural agents
(3).
Water is one of the most powerful agents in erosion. Acting on gravity,
water tears away at the earth and transports the sediment until it no longer
has the energy to do so. When the water reaches sea level or a baseline
related to a lake or another large body of water, the fluvial system loses
its energy and dumps what it has eroded. This is called deposition.
Source: http://math.amu.edu.pl/~sgp/gw/wmd/wmdfig.html |
William Morris Davis described the 'cycle of erosion'
using a stream channel. A young river, such as the diagram at the
above left, is on a gradient. It is fast flowing and on a straight
channel. As centuries pass, the river will erode its channel and
banks, trying to get to the baseline or sea level. A mature river,
sort of the go-between of stages depicts the river reaching base level,
but the river would still erode along its banks, until eventually there
are no banks left, creating a plane. This 'old' stage depicts a river
in a meandering channel, bending its way through the area, trying to find
the easiest route to the sea or another body of water.
However, not all types of material are eroded at the same rate. Different rock properties erode at different rates. Non-resistant rock would erode faster than resisitant rock. This depends mostly on the internal structure of these rock types and the result could be a landform in itself, which will be discussed later. Different grain sizes require different velocities for which erosion can occur. Fine sand (about 0.1 mm), could be eroded at a velocity greater than 10 cm/sec as the Hjulstrom Diagram shows below. |
However,
a boulder (about 100 mm) would require water flowing at least 500 cm/sec
for it to be picked up and transported. Clays also takes about 500
cm/sec to be picked up and transported. This is because clays get
too heavy to be picked up when wet and thus requires a higher velocity
for that grainsize to move. Once these small grain sizes are picked
up however, they cannot be deposited unless the velocity is 0 and even
then, the momentum will keep the clays moving for a period of time.
This diagram assumes that the graisn are well sorted, that the velocities
stay the same, and that they exist on smooth channels.
Erosional Features:
Erosion in a
stream could introduce interesting landforms. In comparing straight
channel and meandering channels, one would notice that both have a thalweg.
A thalweg is a line that connects the deepest points in a stream.
Two erosional features: pools and riffles, occur on this
line. Pools are formed when a convergent flow spreads the riverbed
apart, creating a deeper zone. Riffles are formed in a divergent
flow, where two parts of a riverbed get squished together, forming a shallower
zone.
Pools are the deepest zones on the thalweg while the riffles are the shallowest zone. The depth of the channel increases in a pool, therefore as water moves through this feature, velocity slows in order to compensate and keep the discharge the same. Riffles, on the other hand, have a shallow depth and therefore velocity must increase to compensate. This means that the water in pools can only pick up finer particles and water in riffles would pick up larger material. So why are fine particles found in pools and coarse-grain material found in riffles?
As it happens, when a major rainfall occurs and discharge increases, the velocity of the pool moves faster than that of the riffle. In this way, the coarse-grain material in pools move out and settle in the riffles where the relative velocity is slower. When the discharge returns to normal, fine material will be found in pools and the coarse-grained will be found in riffles.
Potholes:
When pebbles bounce against the bed and the banks of a river, they help erode. For each collision, the pebbles, banks, and riverbed break apart, resulting in transportation downstream and more erosion. After a while, the banks and riverbed look smooth, but this is a slow process as pebbles are warn down as this occurs.
Swirling pebbles along a riverbed will cause scouring
to take place. Eventually a hole will be created and the pebbles, unable
to be pushed out of this hole, will continue to swirl along the hole, eroding,
and deepening it. A pothole results as the hole will continue
to get deeper until the pebbles wear away (4).
Source of photo: http://www.curriculumvisions.com/river/riverNested.html
Waterfalls:
Waterfalls are formed
by erosional means over a sudden drop in height. First, non-resistant
rock gets eroded from underneath resistant rock. Eventually resistant
rock will break off because there is a lack of support under it.
This breakage would create rocks at the bottom a plunge pool which is a
deep area at the base of the fall. Since the resistant rock broke,
the non-resistant rock is now exposed once again. Therefore this
process continues, forcing the waterfall to appear to move backwards over
time until there is no non-resistant rock left (5).
The result is a breathtaking geomorphic feature such as the above photo
of the Canadian Rockies show.
Source of photo: http://www.inge.net/landscapes/canada/page02.shtml
There is little doubt as to why many people are inspired by these landforms. Humans of all walks of life are mesmerized by the idea of water flowing. They are equally struck with awe with the shape the Earth takes because of this powerful liquid. However, to create these landforms, water must use its destructive nature to carve and manipulate the Earth to create what it shows today. Water is never satisfied though, as it will continue to destroy, renew, and give life to this beautiful planet.
Sources:
(1) Strahler A., A. Strahler; Physical Geography: Science and Systems of the Human Environment; John Wiley & Sons, Inc.; United States of America; 1997; pg. 390.
(2) Strahler A., A. Strahler; Physical Geography: Science and Systems of the Human Environment; John Wiley & Sons, Inc.; United States of America; 1997; pg. 362.
(3) http://www.academicpress.com/inscight/06211999/erosion1.htm ; Academic Press.
(4) http://www.curriculumvisions.com/river/riverNested.html ; Brian Knapp; The River Book; Curriculum Visions.
(5) Strahler A., A. Strahler; Physical Geography: Science and Systems of the Human Environment; John Wiley & Sons, Inc.; United States of America; 1997; pg. 400
!Lynx!
