Landforms of Fluvial Erosion + Deposition

Question 1

Waterfall

Answer 1

• change in lithology (granite to limestone), causing differential erosion
• weaker rock erodes faster –> sudden drop, increased height difference.
• plunge pool formed as water flows over waterfall, splashback undercuts resistant rock, eventually collapses.
• retreats upstream.

Question 2

When do rapids occur?

Answer 2

sections of a river where the gradient of river bed is relatively steep.

• increasing turbulence and velocity
• gradient of river steep, bed made up of mainly hard rocks.

Question 3

Potholes

Answer 3

• cylindrical holes drilled into rocky bed of a river by turbulent high-velocity water loaded with pebbles.
• pebbels trapped in slight hollows, water strong enough to drill

Question 4

Braided Channel

Answer 4

when the river is forced to split into several channels separated by islands.

Featured in rivers that are supplied with large loads of sand and gravel.

Question 5

When is a braided channel most likely to occur?

Answer 5

When a river has variable discharges. e.g. Glacial Streams with variable annual discharge.

Question 6

What is the issue with banks formed from sand and gravel?

Answer 6

Easily eroded and generally unstable.

Question 7

What is a consequence of sand and gravel composed banks?

Answer 7

Channel becomes very wide relative to its depth.

River can become choked, with several sandbars and channels that are constantly changing their locations

Question 8

What is a meander?

Answer 8

a bend in a river that form as river sinuosity increases.

Question 9

What does sinuosity mean?

Answer 9

is a measurement of how much a river varies from a straight line.

Its a ratio between the channel length and displacement (straight line distance) between two points in the river’s course.

Question 10

Sinuosity formula

Answer 10

Sinuosity= Channel Length/Displacement

1= perfectly straight river, greater than 1 means there are meanders.

Question 11

How does a meander develop?

Answer 11

When alternating riffles and pools form along a river channel.

Question 12

What is a riffle?

Answer 12

A shallow section of a channel.

Question 13

What is a pool?

Answer 13

a deep section of a channel.

Question 14

Where do riffles and pools develop?

Answer 14

At equal points along the river channel with each pool about 5x the length of the channel.

Question 15

What is the difference between a pool and a riffle?

Answer 15

In a pool the channel is more efficient.

Causes the flow of water to become irregular and the maximum flow is concentrated on one side of the river.

Therefore increases erosion on that side and deposition on the other, causing the channel to bend.

Question 16

What is an Oxbow Lake

Answer 16

an evolution of a meander that underwent extensive deposition and erosion.

Question 17

How is an Oxbow Lake formed?

Answer 17

Strong erosion on the outside bend, strong deposition on inside.

Neck of meander narrows –> in times of extremely high discharge its more efficient for a river to flow across the neck.

When discharge returns to normal levels, river follows new course.

Deposition separates the cut-off from main channel.

Question 18

What is a Levee?

Answer 18

natural embankments produced when a river floods.
When a river floods, it deposits its load over the flood plain due to dramatic drop in the river’s velocity as friction increases greatly.

Question 19

How are raised mounds formed? (Levee)

Answer 19

Largest and heaviest load is deposited first and closest to the river bank, often at the very edge, causing raised mounds.

Question 20

What happens to finer material? (Levee)

Answer 20

Deposited further away from banks causing mounds to appear and taper off.

Repeated floods case mounds to build up and form levees.

Question 21

Are Levees permanent structures?

Answer 21

No, once a river’s discharge exceeds its bankful discharge, the levees can be burst by the high pressure of the water.

Question 22

Can Levees increase the height of the river channel?

Answer 22

Yes so bankful discharge is increased and it becomes more difficult for the river to flood.

Question 23

What is a floodplain?

Answer 23

The area of land around a river channel which is formed during times of flood when the amount of water in a river EXCEEDS its channel capacity and DEPOSITION of rich silt occurs.

Question 24

What is the function of a floodplain?

Answer 24

Is the area the river floods onto when experiencing high discharge.

collects alluvium from load deposition during floods due to reduced efficiency from increased friction.

Question 25

What is alluvium?

Answer 25

is very fertile so floodplains are often used as farmland.

Question 26

what is the width of a floodplain determined by?

Answer 26

the sinuosity of the river and how much meander migration takes place.

Question 27

What does it mean when there is a lot of meander migration?

Answer 27

the area that the river floods on will change and the floodplain will become wider.

Question 28

What is a Delta?

Answer 28

depositional landforms found at the mouth of a river where the river meets a body of water with a lower velocity than the river.

Question 29

What are the requirements for a delta to develop?

Answer 29

the body of water needs to be relatively quiet with a low tidal range so that deposited sediment isnt washed away and has time to accumulate.

Question 30

What are deltas made up of?

Answer 30

3 sediment beds that have been sorted by the size of the sediment.

The bottomset bed, the foreset bed, the topset bed.

Question 31

What is the bottomset bed made out of?

Answer 31

Composed primarily of clay and some other fine grained sediment.

Clay is main because of flocculation where clay meets salt water and the particles clump together and sink due to increased weight.

Question 32

What is flocculation?

Answer 32

is caused by electrostatic charge developing between particles.

Question 33

What is the foreset bed made out of?

Answer 33

composed of coarser sediments.

Not transported very far into stationary body of water and are deposited due to fall in river velocity.

Makes up majority of the delta