2.15 Delta Formation Flashcards
How and why do fluvial landforms form in the tropics?
What are deltas?
- Discrete shoreline protuberances formed where rivers enter oceans or semi-enclosed seas
- Partially subaerial and partially
subaqueous accumulations of river-borne sediments - Formed at the mouths of rivers where the rate of sediment accumulation exceeds the rate of sediment removal by waves and currents
What are the key geomorphological features of deltas?
- Distributaries
- Braided streams
- Alluvial deposits
- Vegetation
- Eyots
- River
- Fine sediment offshore
- Bay
** distributaries, not tributaries
What are the 3 features found across the main parts of deltas?
- Delta plain
- Delta front
- Prodelta
What is a delta plain?
— Located at the top of the delta, above water level
— Flat, often marshy area where river channels branch out and form distributaries
What is a delta front?
Slope area at the edge of the delta plain where sediments are actively deposited
** Steeper gradient than prodelta
What is a prodelta?
— Located seaward of the delta front (low edge of the delta)
— Consists of the finest sediments, which are carried further out into deeper water before settling
— Gently sloping area
— Transitions into the deeper basin floor
What are the 7 processes aiding in delta formation?
- Sediment supply and transport
- Deposition at the river mouth
- Formation of sediment layers
- Progradation of the delta
- Distributary formation and avulsion
- Marine processes and flocculation
- Vegetation and stabilisation
How does SEDIMENT SUPPLY AND TRANSPORTATION aid in delta formation?
— Rivers transport a mixture of sediment from upstream catchment areas to the river mouth
— The amount and type of sediment transported significantly affect the size and shape of the delta
How does DEPOSITION AT THE RIVER MOUTH aid in delta formation?
- As the river reaches its mouth, its velocity decreases significantly because of gradient reduction (gentler).
- Decrease in velocity causes the river to lose its stream capacity and stream competence to transport sediment, resulting in deposition
- Larger, heavier particles are deposited first, closer to the river mouth, while finer particles are carried further out into deeper water.
- Distinct layers of sediment are formed, contributing to the building of the delta.
What is stream capacity?
Amount of particles the river can carry
What is stream competence?
Ability of river to entrain particles
How do FORMATION OF SEDIMENT LAYERS aid in delta formation?
— Bottomset beds (lowest layer)
Where fine particles are carried further offstore and settle in calm, deeper waters.
— Foreset beds (sloping)
Where coarser particles (e.g. sand) are deposited on the steeper front of the delta
— Topset beds (uppermost layer)
On the delta plain, finer materials (e.g. silt, organic matter) are deposited during floods or tidal flows
How does PROGRADATION OF THE DELTA aid in delta formation?
- Continuous deposition of sediments over time
- As more sediment accumulates, there is an outward growth of the delta into the sea or lake (progradation)
- This extends the delta front further into the water body, creating new land areas
How does DISTRIBUTARY FORMATION AND AVULSION aid in delta formation?
- Blockage of the river’s main channel due to accumulated deposited materials at the river mouth
- The river is forced to split into multiple smaller channels (distributaries)
- Periodically, the river may abandon an existing channel in favour of a new, lower path to the sea (avulsion), further shaping its structure.
How do MARINE PROCESSES AND FLOCCULATION aid in delta formation?
Flocculation: Aggregation of fine clay particles due to the mixing of fresh and saltwater (occurs at the delta front)
— The flocculated particles settle and aid in the formation of the prodelta.
Marine processes: Tidal currents, waves, and longshore drift can influence the shape of the delta by redistributing sediment along its edges or eroding parts of the delta front.
How do VEGETATION AND STABILISATION aid in delta formation?
— Plants and vegetation trap additional sediment, slowing down water flow and encouraging further deposition.
— Over time, marshes and swamps may form, contributing to long-term stabilisation and growth of the delta.
- Sediment trapping
- Erosion protection
- Surface building
What are some evidences of delta evolution over time?
— Progradation
— Subsidence
— Sediment yield changes
— Natural diversion of river mouths
When does progradation occur?
When sediment supply is abundant, and the delta advances seaward
What is one specific evidence of progradation?
Some tropical deltas, like those in Java, have recorded progradation rates exceeding 200 meters per year.
What is subsidence?
Sinking of the delta over time due to the weight of the accumulated sediment
— If sediment supply is insufficient to counterbalance the subsidence, the delta becomes prone to submergence and erosion.
What is one specific example of subsidence?
The Mississippi Delta, where subsidence has submerged older subdelta lobes.
What causes sediment yield changes?
Human activities, such as dam construction or deforestation in the river’s drainage basin, can reduce sediment supply to the delta, accelerating erosion.
What are 2 specific examples of the impact of sediment yield changes on deltas over time?
- Nile Delta: After the construction of the Aswan High Dam, there was a reduced supply of sediment to the coast, resulting in significant erosion.
- Mekong Delta: Sediment yield decreased from about 2000 million tonnes per year in the 1960s to the current 400 million per year, though total water discharge has stayed relatively stable. The trapping of sediments by dams and reservoirs accounts for 56% of the total sediment load reduction.
How do natural diversion of river mouths occur?
Abandonment of old delta lobes and creation of new ones over time (natural change in river’s course)
