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Transcript of Delta
Depositional Fluvial Landforms
Delta’s
A delta is a feature of deposition formed when a river flows into a standing body of water
Deposition occurs as the velocity drops on entering the standing body of water, this reducing the capacity and competence of the river bedload and suspended material are ‘dumped’
Delta’s
Delta’s form when velocity drops as the river enters a standing body of water, typically this is at the coast as the river flows into the sea
Delta’s
An example of a delta formed when a river flows into the sea in south west Greenland
Flocculation occurs as fresh water mixes with salt water - e.g where a river flows into the sea -(then called brackish water), and is the process by which tiny particles in suspension like clay and silt (which under normal fresh water conditions would likely never fall out of suspension) coagulate with the salt in the water by chemical reaction and become heavy enough to ‘sink’ and be deposited
Flocculation
Deltas only form where rate of deposition exceeds rate of sediment removal – i.e. Where the critical threshold is no longer met and the system has fallen out of dynamic equilibrium. These conditions are likely where:
The calibre of the load of the river is very large (therefore requiring a high amount of energy to keep in transport)
The costal area into which the river dumps its load has a small tidal range and weak currents meaning there is limited wave action and therefore little subsequent transport of sediment taking place after its initial deposition allowing large deltas to form
Typical conditions for a Delta
Delta’s are usually composed of three types of deposit
Topset beds: The larger and heavier particles are the first to be deposited as the river looses energy
Foreset beds: Medium calibre particles travel a little further before they are deposited as more steep angled ‘wedges’ of sediment
Bottomset Beds: The very finest particles travel the furthest before very low velocity/flocculation leads to their deposition
Composition of Delta’s
Composition of Delta’s
Delta’s can be categorised into three main shapes
Arcuate
Birds Foot
Cuspate
Types of Delta
•The most common
shape of delta
•Characteristics:
curving shoreline
(smoothed by long
shore drift), distinct
pattern of drainage
(branching of
distributaries), and
typically more
‘gravely’ deposits .
•Example: the Nile
Delta
Arcuate Delta’s
•‘Fingers’ of
deposition build
out into the sea
along the
distributaries
channels giving
an appearance
like a birds claw.
Typically with a
finer sediment
•Example: the
Mississippi delta
Birds Foot Delta
•A cuspate delta
is shaped like a
tooth by gentle
but regular
opposing
currents in the
water body the
river flows into,
or longshore
drift
•Example: The
river Ebro delta
in Spain
Cuspate Delta
Web:Image [slide 3]: http://
upload.wikimedia.org/wikipedia/commons/thumb/e/e7/NarsarsuaqGreenland-ArcuateDelta.jpg/380px-NarsarsuaqGreenland-ArcuateDelta.jpg
Image [slide 10] : http://strata.geol.sc.edu/MARINESEDIMENTS/delta/Nile.jpg
Image [slide 11]: http://www.es.ucsc.edu/~es10/classnotes/images/MississippiDelta.drawn.jpg
Image [slide 12]: http://www.iberianature.com/material/photos/google_earth_spain_2/images/delta_de_ebro.jpg
Info: http://wiki.answers.com/Q/Examples_of_cuspate_deltas
Books: AQA text book, Oxford dictionary of Geography
Bibliography