7: Deltas

Question 1

What is a major factor that increases a fluvial system’s sediment supply?

Answer 1

Tectonic uplift

Question 2

Why is there a significantly limited sediment supply for much of Australia’s fluvial system?

Answer 2

Because it does not experience much tectonic uplift

Question 3

What are the 3 categories of processes that affect delta morphology?

Answer 3

Tides, waves and rivers

Question 4

What are the 5 types of delta systems (transgressive) that Colin identified and in what order?

Answer 4

Tidal flat, tide-dominated estuary, barrier estuary, lagoon and strand plain. Order is from tidal dominated to fluvial/river dominated to wave dominated.

Question 5

What are the 4 types of delta systems (prograding/regressive) that Colin identified and in what order?

Answer 5

Tidal flat, tide-dominated delta, wave-dominated delta, strand plain. In order from tide-dominate to river dominated to wave dominated

Question 6

What are the two main things that affect distributary mouth processes?

Answer 6

Salinity and sediment supply

Question 7

What are homopycnal environments characterised by?

Answer 7

Same density of incoming fluvial body with receiving body (likely a lake) and so mixing between them is easy but unorganised

Question 8

What are hypopycnal environments characterised by?

Answer 8

differing density between incoming fluvial body and receiving body (likely marine) which means that the more saline water because it is denser undercuts the fluvial system and so can protrude further up the fluvial river

Question 9

What are hyperpycnal environments characterised by?

Answer 9

incoming fluvial body is carrying significant amount of sediment that it means it is a lot denser and so just falls down along the slope of the delta on its bed

Question 10

What are the Gilbert Systems’ purpose?

Answer 10

They describe what different post-glacial fluvial systems do when flowing in to a lake or the sea

Question 11

What do Homopycnal environment in the Gilbert System experience?

Answer 11

Relative density of the two bodies is very similar. This means that the sediment deposition pattern is fairly plain with coarser sediment deposited first then finer sediment follows.

Question 12

What do hyperpycnal environments in the Gilbert System experience?

Answer 12

When the fluvial system enters a shallow water body. This results in little energy available to transport the sediment and so it is all deposited along the floor close together and packed up but a pattern still exists

Question 13

What do hypopycnal envrionments in the Gilbert System experience?

Answer 13

fluvial system in to a marine environment. Saltwater undercuts the river and increases it s buoyancy so that the sediment is deposited over a broader stretch of the sea floor

Question 14

What are the two main problems/factors affecting the Mississippi Delta? What impact is this having?

Answer 14

Sea level rise and subsidence. Both factors are causing the delta to become broken up and divided very easily.

Question 15

What are the semi-permanent spatial states that the Mississippi Delta constantly finds itself switching between?

Answer 15

Delta lobes

Question 16

What has been used to date the different delta lobes?

Answer 16

Dating of the different identified archaeological artefacts and palaeo dating

Question 17

Describe how a delta lobe is created

Answer 17

During a given time period the fluvial system will continuously aggrade either side of its banks to form a high levee from which it prevents the channel from overflowing and all the sediment thus ends up at the outlet. This consolidation of sediment at the mouth produces a set of sediment islands that divert the channel around it. The channel is in turn eventually diverted elsewhere and so a new delta lobe is created around that new channel

Question 18

What happens to the land/delta behind where the fluvial sediment is deposited on the continental shelf in the Gulf of Mexico?

Answer 18

It experiences subsidence as it is not able to receive any of the sediment that comes from the fluvial system

Question 19

What is the current state of the Mississippi Delta?

Answer 19

It wants to flow out in to a new channel/delta called the Atchafalaya but this is being prevented anthropogenically due to the need to protect the New Orleans’ community and economic potential against the effects of SLR

Question 20

What happens at the mouth when there is a strong river jet and a strong longshore drift?

Answer 20

the longshore drift causes the fluvial sediment being used to form a bar to become angled and carried towards the dominant direction. The strong river jet also however means that the bar is quite flat and spread out

Question 21

What happens at the mouth when there is a strong river jet and weak longshore drift?

Answer 21

The waves assist in the formation of a non-angled cuspate delta

Question 22

What happens at the mouth when there is a weak river jet and strong longshore drift

Answer 22

The longshore drift diverts and angles the formation of a spit at the delta

Question 23

What happens at the mouth when there is a weak river jet and a weak longshore drift?

Answer 23

The waves rework and reshape the consolidation of sediment

Question 24

How is the delta spit formation affected by increasing strength of the longshore drift?

Answer 24

The spit becomes more and more angled/tailored towards the direction of the drift

Question 25

What problem do tide-dominated deltas pose?

Answer 25

They can greatly inundate the delta which is obviously not suitable for settlement purposes but nevertheless people do because of the river nutrient content of these delta types

Question 26

Give and example of an Asian Megadelta

Answer 26

Ganges Brahmaputra

Question 27

What are the Asian Megadeltas a major source of and why?

Answer 27

They have large amounts of minerals, nutrients and resources as well as HEP which means they are heavily anthropogenically utilised.