Definition of Delta
Deltas are an arcuate or fan-shaped feature at the mouth of the river formed by regular deposition of the sediments. The name owes its origin to the Greek letter Δ. The process of formation of a delta begins with the deposition of sediments as soon as the river enters the sea or lake.
First, the river would shed its coarse sediments, which are heavier in comparison to the finer light particles. The finer particles are carried forward by the distributaries to some distance where they come in contact with the saline water, get coagulated and settle. It is clear that delta formation follows a series of steps.
The longitudinal cross-section of the delta shows three depositional units: bottom set bed, foreset bed and top set bed (Gilbert,1890)
a) Bottom set beds: This is a layer of fine material on the bottom of the sea or lake beyond the delta.
b) Foreset beds: They are inclined beds lying between the top set beds and the bottom set beds. They are inclined because the sediments accumulate near the delta front resulting in its sliding from the frontal slope of the delta.
c) Top set beds: They lie on the delta surface and, as the name suggests, lie on the top of all the beds, as mentioned above.
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Conditions for Delta Formation
Delta formation is an outcome of several factors operating together with the variable rates.
Delta formation takes place in those parts of the coast which are relatively protected from high tides and destructive waves. So a comparatively calm sea or lake is a precondition for delta formation.
The second important factor at play is the presence of abundant load in the stream, which can settle at the mouth of the river. The rate of deposition of bedload should exceed the rate of its destruction. When these two conditions appear favourable, delta formation takes place.
Delta is not a stable feature and is regularly modified by the waves, which often redistribute the deposited material at different places along the coast. Further, the fluctuations in the volume of water and the amount of load a river carries also cause modifications in the shape and extent of the delta.
Therefore, there is no uniformity in the shape and size of delta across the world, though arcuate delta is very commonly observed.
The shape of the deltas can be classified into (a) River dominated, (b) Wave dominated, and (c) Tide dominated (Coleman et al., 1986; Galloway and Hobday, 1996)
Given below are a few deltas which have been categorized based on their shape.
Arcuate Delta
These Deltas resemble the arc of a circle. This is the most common shape of the delta found across the world. The distributaries of the main river spread the load in large proportion as they arrive close to the sea, and the deposited material coalesces to give a triangle-like appearance.
The Nile Delta is an ideal example of an arcuate delta. It is also an example of a wave-dominated delta. It has barriers that enclose several lagoons. Niger Delta, Ganga Delta, Rhone Delta, Indus Delta, and Po Delta are all examples of arcuate deltas.
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Cuspate Delta
The cuspate delta gives a pointed tooth-like appearance. It is formed on a straight shoreline where waves are in a predominant position, regularly modifying the deposited sediments by redistributing it along the shoreline. The site of the river meets the ocean and looks like a protrusion in the water. Tiber River forms a cuspate delta at its mouth.
Birdfoot Delta
The delta derives its name from the shape resembling a bird’s claws. The delta has its tributary channels projecting into the sea. The fine load carried by the distributaries is deposited along them in a linear form making small levees. These channels appear to penetrate into the sea in a narrow, finger-like shape.
The Mississippi River is a perfect example of a river-dominated delta. A river in North America forms the grand bird foot delta when it falls into the Gulf of Mexico. This delta type usually forms when a large river joins a relatively shallow gulf or lake.
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Truncated delta, Estuarine delta, and Abandoned delta are examples of other delta types besides these three major types of deltas.
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