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.