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Fluvial Processes – How Rivers Form

Fluvial Processes – How Rivers Form

From the source of a river to its mouth,
the forces of moving water erode and shape the land. We call the
action of flowing water on land fluvial processes. Fluvial processes
can be broken down to the three main components of erosion,
transportation and deposition. For example a river such as the Yangtze
begins its 6300 kilometer journey as melt water from the
glaciers on the Qinghai-Tibet plateau where it erodes mountain sediment. As a
river winds its way eastward across China it erodes and changes the
landscape transporting more material. Eventually the river reaches the sea
where the material is deposited in the Yangtze River Delta. EROSION – Moving water
in the form of rivers are an agent of erosion. River water wears away rocks and
soil found on the riverbed and banks. Rock particles being carried downstream
continue to break down further as they transported by the river. There are four
main forms of fluvial erosion. Hydraulic action –
the force of river water against the banks can cause air to be trapped and
pressured in cracks in the rocks on the riverbank. This continual pressure
causes the rocks to crack further and eventually break away. Abrasion – As the
river transports rocks downstream these rocks wear down the material on the
riverbed. Attrition – Attrition is where the rocks and pebbles being carried by
the river smash against each other. Thereby breaking each other down into smaller,
rounder smoother pebbles. Solution – Solution or corrosion is the chemical
erosion of the rocks of the riverbank. This is because water is slightly acidic
especially where streams flow through rocks like limestone. TRANSPORTATION -Once
it has been eroded, material in the river is transported downstream. How this takes
place depends on the interaction between the velocity of a river and the size of
the particles. As a rule the larger the particle size the higher the
velocity of moving water is required to transport it. When the river’s course is
steep, often near its source, velocities are higher enabling the transportation of
large rocks and boulders. During times of flood energy levels will be even higher.
Transportation takes place in four ways: Solution – This is where the smallest
particles of minerals are dissolved in the water. This usually happens in the middle
and lower reaches of a river, as it takes some time for material to be dissolved. Suspension – Small particles are carried along in the river. This can take place
anywhere along the course of the river. Saltation – Small pebbles bounce along the
riverbed. This mainly takes place in the upper and middle sections of the river. Traction – This is where large boulders
and rocks are rolled along the riverbed. This mainly occurs in the upper reaches
of the river as it is the only place where energy is
high enough to do so. DEPOSITION – When a river loses energy it will deposit
some of the load it is carrying. Energy levels are lowest when rivers meet the sea
or lake and slows down, causing a lot of deposition to take place. Deposition
features such as deltas can form in these places. Deposition also takes place
when the volume of water decreases as happens at the end of a flood or during
a time of drought. Deposition also takes place further up the course of a river. For
example the slower moving water on the inside of a bend of a river will have
less energy and therefore drop its load helping to create a meander.
A major deposition feature of the river is the flood plain in its lower
reaches. This is made up of deposited sand and silt which is known as alluvium.
Floodplains are therefore very fertile and have supported large agricultural
community since ancient times RIVER PROFILES – The long profile is a slice
through the river from the source to the mouth as seen in this diagram. Rivers are
essentially trying to erode to their base level. That is usually sea level or
sometimes a lake that the river may drain into. This is because without the
force of gravity water will not flow. Near the source of a river which is often high
in the mountains, a river will cut vertically to form a steep v-shaped
valley. In these mountainous environments a river will often flow over a series of
rapids and waterfalls. As rivers reach their mid-course they continue to cut
downwards but also cut laterally. Finally as a river near the sea, rivers
flow over flat land and most of the river’s energy is concentrated on cutting
laterally, thereby making the channel wider as it meanders its way towards the
river’s mouth. The cross profile of rivers also changes.
Near the source river channels are narrow and shallow due to the steepness of the
terrain. Rivers in these environments contain many large boulders and rocks.
With all the sediment on the riverbed there is a lot of turbulence and
friction which slows the water down. By the time the river has reached this
lower course the river is flowing fastest until it slows down on meeting
the sea. Here the channels are deep and wide and the banks are the smoothest.

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