7: Deltas Flashcards
What is a major factor that increases a fluvial system’s sediment supply?
Tectonic uplift
Why is there a significantly limited sediment supply for much of Australia’s fluvial system?
Because it does not experience much tectonic uplift
What are the 3 categories of processes that affect delta morphology?
Tides, waves and rivers
What are the 5 types of delta systems (transgressive) that Colin identified and in what order?
Tidal flat, tide-dominated estuary, barrier estuary, lagoon and strand plain. Order is from tidal dominated to fluvial/river dominated to wave dominated.
What are the 4 types of delta systems (prograding/regressive) that Colin identified and in what order?
Tidal flat, tide-dominated delta, wave-dominated delta, strand plain. In order from tide-dominate to river dominated to wave dominated
What are the two main things that affect distributary mouth processes?
Salinity and sediment supply
What are homopycnal environments characterised by?
Same density of incoming fluvial body with receiving body (likely a lake) and so mixing between them is easy but unorganised
What are hypopycnal environments characterised by?
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
What are hyperpycnal environments characterised by?
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
What are the Gilbert Systems’ purpose?
They describe what different post-glacial fluvial systems do when flowing in to a lake or the sea
What do Homopycnal environment in the Gilbert System experience?
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.
What do hyperpycnal environments in the Gilbert System experience?
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
What do hypopycnal envrionments in the Gilbert System experience?
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
What are the two main problems/factors affecting the Mississippi Delta? What impact is this having?
Sea level rise and subsidence. Both factors are causing the delta to become broken up and divided very easily.
What are the semi-permanent spatial states that the Mississippi Delta constantly finds itself switching between?
Delta lobes
What has been used to date the different delta lobes?
Dating of the different identified archaeological artefacts and palaeo dating
Describe how a delta lobe is created
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
What happens to the land/delta behind where the fluvial sediment is deposited on the continental shelf in the Gulf of Mexico?
It experiences subsidence as it is not able to receive any of the sediment that comes from the fluvial system
What is the current state of the Mississippi Delta?
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
What happens at the mouth when there is a strong river jet and a strong longshore drift?
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
What happens at the mouth when there is a strong river jet and weak longshore drift?
The waves assist in the formation of a non-angled cuspate delta
What happens at the mouth when there is a weak river jet and strong longshore drift
The longshore drift diverts and angles the formation of a spit at the delta
What happens at the mouth when there is a weak river jet and a weak longshore drift?
The waves rework and reshape the consolidation of sediment
How is the delta spit formation affected by increasing strength of the longshore drift?
The spit becomes more and more angled/tailored towards the direction of the drift
What problem do tide-dominated deltas pose?
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
Give and example of an Asian Megadelta
Ganges Brahmaputra
What are the Asian Megadeltas a major source of and why?
They have large amounts of minerals, nutrients and resources as well as HEP which means they are heavily anthropogenically utilised.
