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Deltas are features of deposition, located at the mouth of the river (so in the lower course) as it
enters a sea or lake. They are made up of three layers: topset, foreset and bottomset.
1. Bottom set: Have finest material, carried furthest in suspension and sinks to sea bed.
2. Foreset: More readily deposited, coarser material. Builds out to sea.
3. Topset: Deposits from river with the most coarse material.
Deltas form where inputs of material exceed the rate of removal by coastal currents and longshore
drift as the river's ability to carry such large amounts of load decreases (due to velocity
decreasing as the river enters the sea or lake due to the vast new space for the water), deposition
becomes the main process.
As freshwater mixes with seawater at the mouth, coagulation and flocculation occur where
negative charges on clay particles interact electrolytically with saltwater allowing clay particles to
attract and clump together. These larger floccules then settle sooner than individual clay particles.
The finest clay is taken furthest away from the coast and eventually deposited as the bottomset
bed, followed by coarser sediment building on top of it as a foreset bed at an inclined angle, this
extends the delta. The coarsest material deposits on the top as the topset bed. It causes outward
migration of the delta into the sea.
Deltas are dynamic environments depending on a balance between sedimentation and sea level,
over many years the position of the main channel can change: "avulsion".
There are three types of delta:
1. Bird foot delta: seawater is denser than freshwater allowing fluvial sediments to be carried far
away by floating on top of the underlying seawater. Most deposition occurs on the river margins
building levees, these are broken during floods creating distributaries, these can add sediment and
build the delta up in many directions. They are spread over a wide area. E.g. Mississippi
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2. Arcuate delta: these form where the waters are more mixed thus similar densities are created
(so material deposits quicker) and there is stronger wave action these are usually close to the
coastline and less indented, rounded with convex edges. E.g. Ganges.
3. Cuspate delta: strong waves make a cuspate delta a thinner, tooth-shaped version of the
arcuate.It is shaped by gentle, regular but opposing sea currents or longshore drift. E.g. Ebro,
Factors influencing delta formation:
Amount and type of sediment available.…read more
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Deltas are especially favourable for agriculture, due to the deposition of fine sediment, but
at the same time are places of high flood risk as shown by the Ganges Delta.…read more