Dynamic Surface L8 Flashcards
What are the 3 functions of fluvial systems?
Erosion
Transportation
Deposition
What do fluvial deposits record?
Flow
Climate
Tectonics
Sea level
What happens to flow velocity and bedload size as you move downstream
Flow velocity and bedload size decrease downstream
What areas of the globe contribute the highest amount of sediment to the oceans?
Tectonically active regions / new mountain chains (e.g. Himalayas/Andes)
What do alluvial fans indicate?
Alluvial fans indicate sharp terrestrial relief as they form at significant changes in slope.
They also indicate climatic extremes.
What is the intersection point of an alluvial fan?
Where the channel meets the fan surface
What is the depositional lobe of an alluvial fan and where is it located?
The depositional lobe is the point at which the alluvial fan loses velocity and deposits sediment located below the intersection point.
What is an intramontane basin?
A basin formed between mountains
What is a sediment apron / bajada?
Aa coalescence of several fans
What is the size of a fan determined by?
Size of drainage basin
Relief of drainage basin
Lithology
Climate
What is the relationship between the size of an alluvial fan and the gradient of its surface?
The bigger the fan, the lower the gradient of its surface
What does the gradient of an alluvial fan depend on?
The proportion of cohesive fines in the sediment
e.g. more clay + silt = steeper the gradient
What happens to grain size, bed thickness and channel depth as you move from the apex to the toe of the alluvial fan?
Grain size, bed thickness and channel depth all decrease
What happens to grain sorting as you move from the apex to the toe of the alluvial fan?
Grain sorting increases
What happens to the divergence of flow from the apex to the toe of the alluvial fan?
Increasing divergence of flow
What is a debris flow ‘dry’ fan?
Steep and conical fans formed when lots of debris falls down a confined canyon and onto the fan surface every few hundred years.
How can you distinguish between the younger and older sediments of debris flow fans?
The younger sediment will be paler as it has had less time to oxidise
The older sediment will be darker and more oxidised
What is the main feature of a debris flow fan that helps to distinguish it?
Clasts are matrix supported in a debris flow fan
Is the sediment in debris flow fans well or poorly sorted?
Poorly sorted
What is the general shape of clasts in a debris flow fan?
Sub angular
What is a sheet-flood ‘wet’ fan?
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.
In which direction does the sediment in sheet-flood fans fine?
Sediment fines upwards
Are sheet-flood fans clast supported or matrix supported?
Clast supported
What is a stream-channel fan?
Channel systems that evolve on the surface of the fan in times of high rainfall, often forming cross bedding.
Where are braided rivers found?
Braided rivers are common in high altitude regions of high gradient and a high sediment supply.
Why are braided rivers often seasonal?
Ice melt from glaciers in the mountains greatly alternate the river flow.
What is formed when coarser sediment is deposited in a braided river?
Coarser sediment is deposited into elongated bars in the downstream direction, splitting up the channel
What are 2 differences between sand and gravel deposited by braided rivers?
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
What type of bedding do channel deposits in braided rivers produce and why?
Trough cross bedding as flow is moving faster
What type of bedding do bars in braided rivers produce and why?
Planar cross bedding as flow is moving slower
Where do sinuous ‘meandering’ rivers form?
Lowlands
Where does erosion and deposition occur in a sinuous river?
Erosion occurs on the outside of the bend
Deposition occurs on the inside of the bend
Where is the channel deepest in a sinuous river?
At the outside of a bend
Where is the channel shallowest in a sinuous river?
At the inside of a bend
In which direction does the bedding of a sinuous river fine?
Upwards
What is a levee?
Slightly elevated banks of a river that form when sediment is pushed aside by the river.
What is a crevasse splay?
Form when levees break causing fans of coarser grained sediment to empty out onto a flood plain.
What is the main difference between sinuous and braided rivers?
In braided rivers you almost never see a complete channel whereas in sinuous rivers complete channels are clear.
