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Deltas are divided into following six types on the basis of shape and growth: 1. Arcuate Delta 2. Bird-Foot Delta 3. Estuarine Delta 4. Truncated Delta 5. Growing Delta 6. Blocked Delta.
Type # 1. Arcuate Delta:
Such deltas are like an arc of a circle or a bow and are of lobate form in appearance wherein middle portion has maximum extent towards the sea whereas they narrow down towards their margins. Such deltas are formed when the river water is as dense as the sea water. The arcuate or semi-circular shape is also given to such deltas by sea waves and oceanic currents.
The Nile Delta is the best example of arcuate deltas (fig. 18.17), which is also called as Nile type of delta. Arcuate deltas are formed of coarser materials including gravels, sands and silt. The main river is bifurcated into numerous channels known as distributaries. Such deltas are very often formed in the regions of semi-arid climate.
Significant examples of arcuate delta include Ganga delta, Rhine delta, Niger delta, Yellow (Hwang Ho) delta, Irrawaddy delta, Volga delta, Indus delta, Danub Delta, Meekong Delta, Po delta, Rhone Delta, Leena delta etc. Arcuate delta is an example of growing delta as it grows towards the sea every year but the annual rate of growth varies from one delta to another. This process of seaward growth of deltas is called progradation.
Type # 2. Bird-Foot Delta:
Bird-foot deltas resembling the shape of foot of a bird are formed due to deposition of finer materials which are kept in suspension in the river water which is lighter than the sea water. The rivers with high velocity carry suspended finer load to greater distances inside the oceanic water.
The fine materials after coming in contact with saline oceanic water settle down on either side of the main channel and thus a linear delta is formed. It is interesting to note that the distributaries of the main channel also form linear segments of delta. These linear bars of sediments on either side of the distributaries of the main channel resemble the fingers of human hand. Such delta is, thus, also called finger delta. The Mississippi delta exhibits the best example of bird-foot delta (fig. 18.20).
Type # 3. Estuarine Delta:
The deltas formed due to filling of estuaries of rivers are called estuarine deltas. Those mouths of the rivers are called estuaries which are submerged under marine water and sea waves and oceanic currents remove the sediments brought by the rivers. There is continuous struggle between the rivers and sea waves wherein the former deposit sediments while the latter remove them. Whenever rivers succeed in depositing sediments at their submerged mouths, long and narrow deltas are formed.
Such deltas are called estuarine deltas. The deltas of Narmada and Tapi (formerly Tapti) rivers of India are the examples of estuarine deltas. The other significant examples of estuarine deltas include Mackenzie delta, Vistuala delta, Elb delta, Ob delta, Seine delta, Hudson delta etc.
Type # 4. Truncated Delta:
Sea waves and ocean currents modify and even destroy deltas deposited by the river through their erosional work. Thus, eroded and dissected deltas are called truncated deltas.
Type # 5. Blocked Delta:
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Blocked deltas are those whose seaward growth is blocked by sea waves and ocean currents through their erosional activities. The progradation of deltas may also be hampered due to sudden decrease in the supply of sediments consequent upon climatic change or management of catchment areas of concerned rivers.
Type # 6. Abandoned Delta:
When the rivers shift their mouths in the seas and oceans, new deltas are formed, while the previous deltas are left unnourished. Such deltas are called abandoned deltas. The Yellow (formerly Hwang Ho) river of China has changed its mouths several times and thus has formed several deltas.
For example, the present delta of the Yellow river is to the north of Shantung Peninsula while the previous delta was deposited to the south of the peninsula. The western part of the Ganga delta, which is drained by the Hoogli River is an example of abandoned delta.