Dynamic Surface L8

Question 1

What are the 3 functions of fluvial systems?

Answer 1

Erosion
Transportation
Deposition

Question 2

What do fluvial deposits record?

Answer 2

Flow
Climate
Tectonics
Sea level

Question 3

What happens to flow velocity and bedload size as you move downstream

Answer 3

Flow velocity and bedload size decrease downstream

Question 4

What areas of the globe contribute the highest amount of sediment to the oceans?

Answer 4

Tectonically active regions / new mountain chains (e.g. Himalayas/Andes)

Question 5

What do alluvial fans indicate?

Answer 5

Alluvial fans indicate sharp terrestrial relief as they form at significant changes in slope.

They also indicate climatic extremes.

Question 6

What is the intersection point of an alluvial fan?

Answer 6

Where the channel meets the fan surface

Question 7

What is the depositional lobe of an alluvial fan and where is it located?

Answer 7

The depositional lobe is the point at which the alluvial fan loses velocity and deposits sediment located below the intersection point.

Question 8

What is an intramontane basin?

Answer 8

A basin formed between mountains

Question 9

What is a sediment apron / bajada?

Answer 9

Aa coalescence of several fans

Question 10

What is the size of a fan determined by?

Answer 10

Size of drainage basin
Relief of drainage basin
Lithology
Climate

Question 11

What is the relationship between the size of an alluvial fan and the gradient of its surface?

Answer 11

The bigger the fan, the lower the gradient of its surface

Question 12

What does the gradient of an alluvial fan depend on?

Answer 12

The proportion of cohesive fines in the sediment
e.g. more clay + silt = steeper the gradient

Question 13

What happens to grain size, bed thickness and channel depth as you move from the apex to the toe of the alluvial fan?

Answer 13

Grain size, bed thickness and channel depth all decrease

Question 14

What happens to grain sorting as you move from the apex to the toe of the alluvial fan?

Answer 14

Grain sorting increases

Question 15

What happens to the divergence of flow from the apex to the toe of the alluvial fan?

Answer 15

Increasing divergence of flow

Question 16

What is a debris flow ‘dry’ fan?

Answer 16

Steep and conical fans formed when lots of debris falls down a confined canyon and onto the fan surface every few hundred years.

Question 17

How can you distinguish between the younger and older sediments of debris flow fans?

Answer 17

The younger sediment will be paler as it has had less time to oxidise
The older sediment will be darker and more oxidised

Question 18

What is the main feature of a debris flow fan that helps to distinguish it?

Answer 18

Clasts are matrix supported in a debris flow fan

Question 19

Is the sediment in debris flow fans well or poorly sorted?

Answer 19

Poorly sorted

Question 20

What is the general shape of clasts in a debris flow fan?

Answer 20

Sub angular

Question 21

What is a sheet-flood ‘wet’ fan?

Answer 21

A lower relief, semi-circular fan produced when intense rainfall causes flash flooding in the feeder channel resulting in sediment-laden water spreading across the fan surface.

Question 22

In which direction does the sediment in sheet-flood fans fine?

Answer 22

Sediment fines upwards

Question 23

Are sheet-flood fans clast supported or matrix supported?

Answer 23

Clast supported

Question 24

What is a stream-channel fan?

Answer 24

Channel systems that evolve on the surface of the fan in times of high rainfall, often forming cross bedding.

Question 25

Where are braided rivers found?

Answer 25

Braided rivers are common in high altitude regions of high gradient and a high sediment supply.

Question 26

Why are braided rivers often seasonal?

Answer 26

Ice melt from glaciers in the mountains greatly alternate the river flow.

Question 27

What is formed when coarser sediment is deposited in a braided river?

Answer 27

Coarser sediment is deposited into elongated bars in the downstream direction, splitting up the channel

Question 28

What are 2 differences between sand and gravel deposited by braided rivers?

Answer 28

Sand is deposited further downstream whereas the gravel is deposited upstream

Deposited gravel forms elongated bars in the direction of flow whereas deposited sand is transverse to the direction of flow

Question 29

What type of bedding do channel deposits in braided rivers produce and why?

Answer 29

Trough cross bedding as flow is moving faster

Question 30

What type of bedding do bars in braided rivers produce and why?

Answer 30

Planar cross bedding as flow is moving slower

Question 31

Where do sinuous ‘meandering’ rivers form?

Answer 31

Lowlands

Question 32

Where does erosion and deposition occur in a sinuous river?

Answer 32

Erosion occurs on the outside of the bend
Deposition occurs on the inside of the bend

Question 33

Where is the channel deepest in a sinuous river?

Answer 33

At the outside of a bend

Question 34

Where is the channel shallowest in a sinuous river?

Answer 34

At the inside of a bend

Question 35

In which direction does the bedding of a sinuous river fine?

Answer 35

Upwards

Question 36

What is a levee?

Answer 36

Slightly elevated banks of a river that form when sediment is pushed aside by the river.

Question 37

What is a crevasse splay?

Answer 37

Form when levees break causing fans of coarser grained sediment to empty out onto a flood plain.

Question 38

What is the main difference between sinuous and braided rivers?

Answer 38

In braided rivers you almost never see a complete channel whereas in sinuous rivers complete channels are clear.