1.3 River channel processes and landforms

Question 1

What is the process of erosion

Answer 1

• Erosion is the process of the wearing away of soil and rock
• In a river channel, both the force of water as well as fragments of rock cause the river bank to wear away in certain areas over time

Question 2

Explain the processes of erosion: corrasion

Answer 2

• This is the process of rocks scraping and grinding (known as abrasion) along the river channel, causing soil and rock to wear away
• This usually causes the river bed to deepen and widen, and is especially prominent when a river is flowing with high velocity (like during a flood) as it has enough energy to carry larger rocks

Question 3

Explain the processes of erosion: solution

Answer 3

• This is the process of water dissolving parts of rock/soil that makes up the river channel
• Solution is most prominent when the river channel is made from water soluble rocks, such as chalk, gypsum, or limestone
• Water can slowly wear away at the river channel over time through this process.

Question 4

Explain the processes of erosion: hydraulic action

Answer 4

• Hydraulic action is the sheer force of water can cause rocks to fragment off when the channel is hit

Question 5

Explain the processes of erosion: cavitation

Answer 5

• Cavitation is where water forces its way into small cracks, causing the air in these spaces to be compressed and put under pressure, widening the cracks and fragmenting the rocks

Question 6

Explain the processes of transportation: traction

Answer 6

• The movement of larger rocks and pebbles through water rolling them along the river bed
• As the rocks are heavier, they cannot be carried by water within the channel as the river does not have enough energy.

Question 7

Explain the processes of transportation: saltation

Answer 7

• Pebbles are bounced along the riverbed

Question 8

Explain the processes of transportation: suspension

Answer 8

• Small pebbles and material are carried (suspended) within the water, rather
  than rolling/bouncing along the river bed.

Question 9

Explain the processes of transportation: solution

Answer 9

• Soluble materials are carried within the water

Question 10

Explain the process of deposition or sedimentation

Answer 10

• Deposition is where the river drops its load when the river no longer has enough energy to carry the load
• Deposition may occur when a river becomes shallower or when the volume of water decreases.

Question 11

What is the Hjulstöm Curve?

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Answer 11

• The Hjulström Curve is a graph that shows how the velocity of a river affects the river’s material
• The relationship between material size and velocity is presented, and the graph shows whether the material will be eroded, transported, or deposited based on its size and the river velocity.

Question 12

What are the main points to take from the Hjulström Curve: critical erosion velocity curve

Answer 12

• The critical erosion velocity curve is the minimum velocity needed for the material to be picked up and then eroded
• The velocity must reach this line for the material to be transported initially, but once the velocity exceeds this line (in the green area), there is a likelihood that the particle will be eroded.

Question 13

What are the main points to take from the Hjulström Curve: relationship between energy and material size

Answer 13

• Overall, the critical erosion velocity curve shows the relationship between material size and the energy (velocity) needed to transport and erode it
• In general, the larger the material size, the more energy needed to transport/erode, therefore the higher the velocity needed
• This is not true of clays and silts, because these bond together, meaning it is harder to break the particles up to transport/erode them. (notice how the critical erosion velocity is much higher for these smaller particles than sand)

Question 14

What are the main points to take from the Hjulström Curve: settling velocity curve

Answer 14

• The settling velocity curve is the point at which the material no longer has the energy required to be transported, meaning it is deposited
• If the velocity falls below this line into the blue area, there is a likelihood that the particles will be deposited
• The larger the material, the higher velocity needed to keep the particles transporting.

Question 15

What are the main points to take from the Hjulström Curve: clays and sands

Answer 15

• There is no settling velocity for clays and silts, as once they are transported, they will continue to be transported, and will not be deposited.

Question 16

Explain river flow: laminar flow

Answer 16

• The flow of water occurs in parallel streams
• There are no croos currents or swirls
• It is one directional and has orderly flow

Question 17

Explain river flow: turbulent flow

Answer 17

• There is disorderly flow
• There are changes in velocity
• This type of flow can be caused by friction, especially when river channels are irregular
• Eddies, which are swirling reverse currents, are common in this type of flow

Question 18

Explain river flow: helicoidal flow

Answer 18

• It has a corkscrew movement (spiral)
• It occurs in bends in the river, called meanders
• They are responsible for the erosion and deposition in a meander

Question 19

Explain river flows: what is thalweg

Answer 19

• The line of fastest flow (maximum velocity) within a river
• It is also the deepest point within the river channel
• This is because erosion is greatest when the stream flow is the fastest

Question 20

Explain channel types: straight

Answer 20

• Mainly occur in the upper course of the river
• They are single, straight channel
• Here vertical erosion is prevalent
• Thalweg moves from side to side

Question 21

Explain channel types: meandering

Answer 21

• Occurs in the middle and lower course of the river
• It is a series of bends and curves from side to side
• Deposition occurs on the inside of the bend
• Erosion occurs on outside of the bend

Question 22

Explain channel types: braided

Answer 22

• Large deposits of sediment within the channel, causing multiple channels separated by islands of sediment
• Usually occur where discharge fluctuates
• River deposits when it loses energy
• Do not usually occur in the upper course

Question 23

Explain river landforms: waterfalls

Answer 23

• In an area where a river flows over an area of hard rock and soft rock, the soft rock erodes more quickly. This causes the soft rock to erode away beneath and underneath the hard rock, creating a step.
• Over time, the soft rock continues to erode further, undercutting the hard rock. The hard rock is not eroded as quickly, leaving it suspended in the air as an overhang. The rotational movement of the water quickens erosion, creating a deep plunge pool
• Due to the force of gravity, the unsupported overhang collapses. The broken up rocks fall into the plunge pool, which act as tools for erosion and further deepens the plunge pool.
• Erosion also continues to undercut underneath the hard rock, creating an overhang again.
• The continual process of the overhang collapsing causes the waterfall to retreat upstream over time. The plunge pool continues to deepen, and the hard rock continues to be undercut to create an overhang.

Question 24

Explain river landforms: gorges

Answer 24

• A gorge is a steep sided, narrow valley with a river flowing through it
• A gorge is formed when a waterfall continually retreats over time. The back of a waterfall wall and the channel floor is eroded, whereas the valley sides are not, leaving a deep gorge when the waterfall retreats upstream.

Question 25

Explain river landforms: meanders

Answer 25

• The thalweg is located on the outside bend of a river. This is the line of fastest flow
• Erosion is stronger on the outside of a bend, as this is where the river is at its highest velocity. The river channel is at its deepest here, meaning there is less friction.
• Deposition is more prominent on the inside of a bend, as this is where the river is at its lowest velocity. Therefore, it has less energy to carry the material and instead deposits it
• The river channel is at its shallowest here, causing the water to be slower due to friction.
  Over time, erosion causes the outside bends to get closer, until only a small area of land remains (called the neck).

Question 26

Explain river landforms: explain the cross section of a meander

Answer 26

• Within the river channel of a meander, different landforms occur as a result of the erosional and depositional processes
• River cliff: A cliff created when the bank is undercut by erosion on the outside of the bend
• Point bar: A deposit of fine sediment, such as sand, on the river bank inside of a meander bend
• The point bar usually changes position as the meander changes shape, leaving a mark where it once was
• Slip off slope: sand or shingle desposited on the inside of the bend

Question 27

Explain river landforms: oxbow lakes

Answer 27

• When a river is in flood and it breaks its banks, the river will flow along the shortest available course
• In some meanders, this will be across the neck, cutting off the meander
• Eventually, the meander bend becomes fully cut off due to deposition. This forms an oxbow lake.

Question 28

Explain river landforms: bluffs and how are they formed

Answer 28

• A bluff is a large, rounded cliff on the side of a river
• Bluffs are formed on the outside bend of a meander, where erosion is dominant
• The water erodes the lower section of the river bank, causing the upper section of the river bank to become unsupported and therefore unstable
• Eventually, the unsupported upper part of the bank collapses, causing a steep bluff to be left
• Bluffs also occur when water in the outside bend of a meander erodes a steep valley wall. This occurs on a wide flood plain, when a meander (over thousands of years) shifts from side to side to reach the valley wall.

Question 29

Explain river landforms: riffle and pool sequences

Answer 29

• Riffle-pool sequences are alternating patterns of shallow and deeper water in a river channel, common in rivers with gentle slopes
• These are formed because a river tends to adjust its course of flow to efficiently transport its load downstream (using as little energy as possible).
• Riffle: The higher, shallower area of the sequence, characterised by deposits of material
• Pool: The lower, deeper area of the sequence.

Question 30

Explain river landforms: floodplains

Answer 30

• A floodplain is a large, flat area of land near a river, that forms due to erosion and deposition
• Over time, meanders can erode a v-shaped valley into a wider, flat valley
• When a river is in flood, the sediment that is transported within the river leaves the channel and spreads out onto the river bank
• The water loses energy when it is no longer flowing through its channel, meaning the sediment is deposited onto the flat plain.
  Over time this deposition builds up, making it higher. The soil is usually very fertile.

Question 31

Explain river landforms: levees

Answer 31

• Levees are natural embankments along a river that build up due to repeated flooding. They create raised edges on the river bank, making it less likely for the river to break its banks.
• When a river floods, the sediment being transported in the river floods onto the floodplain.
• The heavy, coarse material is deposited first (closest to the river channel), because it requires the most energy to be transported.
• The finer, light sediment (such as sands and silts), require less energy to be transported, so it can be transported and deposited further away from the channel. This creates a tapered shape
• Over time, the levee becomes larger and more well-formed. River levels can be higher without breaking the bank and flooding; the levees act as natural flood defences.

Question 32

Explain river landforms: deltas

Answer 32

• Deltas form at the mouth (end) of a river, where the river carries a large amount of sediment
• When a river meets a body of water (a lake or an ocean) there is a large difference in velocity between the high velocity river and the low velocity water body
• This causes the river to mass deposit sediment, causing a series of sediment islands with smaller streams and rivers flowing through them.