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The Features and Processes of a River Along Its Profile

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The Features and Processes of a River Along Its Profile



Introduction
------------

Along the path of a river, from source to mouth, the river shows many
different features and is affected by several different processes.
These processes are going to be described and explained in the course
of this essay and diagrams will be used to back-up and justify my
ideas.

A river can be simply divided into an upland or lowland river
environment. Upland features and processes occur at a higher altitude,
closer to the source of the river; lowland features and processes
occur at lower altitudes near the mouth of a river. A river's course
can be better divided into a young, mature and old stage.

A simple map of a river showing the different stages is shown below:

The main processes acting upon a river are:

* Erosion

* Transportation

* Deposition

These three main river processes can be further sub-divided into
"sub-processes". Erosion, for example, wears rocks away through 4
sub-processes:

* Corrasion or abrasion

* Attrition

* Corrosion or erosion by solution

* Hydraulic action

Transportation can be further divided into:

* Traction

* Saltation

* Suspension

* Solution

A description of these processes and diagrams showing how they affect
the river are shown below:


Erosive Processes

Corrasion or abrasion

This is when smaller material which is suspended, rubs against the
river banks. This is more common in lowland areas, where the river is
at a low altitude and has a low gradient.

Attrition

This is when boulders are being transported along the river bed
collide and smash up into smaller pieces. This is likely to occur
where river velocity is high in upland areas.

Corrosion or erosion by solution

This is when acids in the water dissolve soft rocks such as limestone.

Hydraulic action

This is when the sheer force of the river breaks up and dislodges
particles from the rivers banks and bed.


Transportation Processes

Traction

Saltation

Suspension

Solution

As well as these river processes, other processes also affect and
change the landscape of a river. These processes include weathering
and the effect of humans. More information on this later!

In a young river valley the main features found include:

* Narrow V-shaped valleys and Interlocking Spurs

* Rapids and Waterfalls

In an older river the main features found include:

* Meanders and ox-bow lakes

* Floodplains and Levees

* Deltas

As mentioned earlier, these features do not just appear, they are
formed by different processes occurring at different stages of the
river.


A) Features and Processes of Upland River Environments
------------------------------------------------------


(i) Narrow V-Shaped Valley and Interlocking Spurs

Description

In its youth the river valleys are steep-sided, narrow and shaped like
the letter "V". A young river is often forced to wind around
protruding hillsides - these are called interlocking spurs.

Explanation

Near its source the river has lots of spare energy to transport large
boulders and stones along the bed. This causes the river to cut down
rapidly - a process called vertical erosion. When the young river has
lots of energy it can also lengthen its channel by cutting backwards -
a process known as headward erosion.

The valley sides are steep due to soil and loose rocks being washed
downhill after heavy rainfall. This material is then added to the load
of the river.


A diagram showing interlocking spurs


(ii) Rapids and Waterfalls

Description

A Waterfall is when a river meets a sudden interruption in the river
bed and its course.

Explanation

Waterfalls form when a sudden drop is met in the course of a river.
This drop may be a result of a drop in sea level, erosion by ice or
earth movements. But usually a waterfall is formed when a river meets
a layer of harder rock, then flows over a layer of softer rock. The
softer rock underneath is eroded more quickly than the more resistant
hard rock, resulting in an undercutting below the harder rock. Over
time, as the undercutting increases, an overhang of hard rock is
formed, and is subsequently collapses under the force of its own
weight. Over time, the break-offs from this collapse will form a deep
plunge pool. This is because of the rocks being swirled around
(especially at times of high discharge) and colliding with the sides
of the pool. These types of erosion are called hydraulic action and
attrition. After these processes are repeated several times, a
steep-sided gorge is formed as the waterfall retreats. Rapids occur
where the layers of hard and soft rock are very thin, so no break in
the slope occurs like in a water fall.


A diagram showing Rapids and Waterfalls


B) Features and Processes of Lowland River Environments
-------------------------------------------------------


(i) Meanders and Ox-bow lakes

Description

A Meander is a bend in a river. The bend increases over time to form
an ox-bow lake, which is shaped like a banana.

Explanation

When a river is nearing its mouth, it is flowing over much flatter,
with less of a gradient. At this point in the rivers profile, large
bends often form, known as meanders. These meanders form when a piece
of hard rock lies on the bank on one side of a straight river. As the
water flows past the hard rock, the soft rock past it is eroded. This
directs the flow toward the outside of the river, causing lateral
erosion on one side of the river, causing a slight bend in the path.
This slight bend is exaggerated as the water is thrown to the outside
of the bend due to the centrifugal acting upon the water. So as the
river approaches the meander, the flow is directed toward the outside
of the bend, causing erosion on one side of the meander. This causes a
reduction in friction, increasing the velocity of the river at this
point. This velocity enables the river to transport more material in
suspension, and this material erodes in the form of corrasion against
the bank. This causes an undercutting of the bank, causing it to
collapse, and retreat backwards like a waterfall would. This
undercutting is called a river cliff'.

During this increase in velocity on the outside of the bend, the
velocity decreases on the inside of the bend. This is because there is
less water on the inside because it has been thrown to the outside of
the meander loop. Furthermore, the velocity decreases because the is
more friction against the river bed and bank on the inside of the
meander. Due to this loss in velocity and energy, the material in
suspension (the rivers load) is deposited. This deposition builds up
to form a slip-off slope.

As the erosion continues on the outside of the meander loop, the
meander neck narrows until, often during flooding or high discharge,
the necks join, meaning the river can flow a shorter course. The
original meander will be left as a pool of water, known as an ox-bow
lake. This lake will gradually dry up, unless there is heavy rainfall.


A diagram showing Meanders and Ox-bow lakes


(ii) Floodplain and Levées

Description

A floodplain is an area of flat land surround a river. It is formed
when the river floods. A levee is also formed when a river floods, and
is a natural embankment on the upper banks of a river.

Explanation

A river is widened by the lateral erosion of the river. When there is
high discharge, the high amounts of energy in the river can transport
large amounts of material in suspension. In high discharge, the river
often floods, spreading out across any nearby flat land. This land
causes sudden increase in friction, and so velocity decreases causing
much deposition of fine silt. Every time a river floods, another thin
layer of silt is deposited, and a flat floodplain is formed. The
heaviest material will be deposited first when a river floods, forming
an embankment next to the river, called a levees. This levees acts as
a natural flood defence, and are sometimes strengthened. If these
levees are broken, then huge floods occur over great areas of land.


A diagram showing Floodplains and Levees


(iii) Deltas

Description

A delta is a small area of land formed from deposition when a strong
current meets a weaker current.

Explanation

As a river gets closer to its mouth and the sea, the current is
dramatically reduced, and its material deposited. This deposited
material can block the rivers path. The river must then divide into a
series of channels, called distributaries, in order to reach the sea.
Over time, the deposited material builds up to form a delta. These are
only formed when the amount of material brought down is too much for
the sea currents to remove. A delta can also form when a river flows
into the gentle, slow current of a lake.


A diagram showing Deltas


Conclusion

As mentioned earlier, a river has a variety of features along its
length depending on:

* The energy that a river has to erode and transport its material -
a river will have more energy near its source when it is young and
fast-flowing.

* the gradient of a river - the steeper the gradient, the faster the
river flows and the more energy it has to erode and transport

* the shape of the river valley and valley sides - the steeper and
deeper the valley the faster the river will flow unless the water
is too turbulent to flow forwards

Other factors affect the shape of a river and the valley sides
including:

* Weathering

* Human action

Weathering

This is when wind, rain and temperature changes act upon the valley
sides. The effect of weathering is to break up the rock or soil on the
valley sides which then falls into the river. This "extra" material is
then used by the river in abrasion or adds to the "load" of the river.

Two important types of physical weathering include:

* freeze-thaw

* exfoliation

Freeze-thaw occurs when water within cracks in a rock freezes and
expands and then warms up and melts. Over time this can break up the
rock into scree, which under the influence of gravity rolls down the
valley sides into the river. This process would affect Britain in the
winter months.


Exfoliation or "onion" weathering

This occurs when a rock heats up in the sunshine and then cools down
at night. Over time the rock splits into layers and breaks up and
rolls into the river. This process occurs in Britain during the summer
months.

If weathering continues valley sides will be worn down and more load
will be added to the river.

Human action

This can affect rivers in many ways. Humans can alter the shape and
course of a river by straightening the channel, building artificial
banks, dredging the channel and building bridges, viaducts and locks.
Humans also erode the sides of rivers by farming too close to the
river's edge and walking along paths which follow a river. Humans can
also pollute a river by dumping waste and chemicals into the water,
killing fish and vegetation.

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