Delta a deposit of clay, silt and sand formed at the mouth of a river where the stream loses velocity and drops part of its sediment load. No delta is formed if the coast is sinking or if there is an ocean or tidal current strong enough to prevent sediment deposition. Coarse particles settle first, with fine clays last and found at the outer regions of the delta. This process of building up is complex, and leads to the formation of a number of separate channels and distributaries, isolated lagoons, marshy land, and a network of small creeks. Most deltas are complicated and multiple, but in a simple delta three main types of bed may be distinguished: bottomset beds, foreset beds and topset beds. Named after the Greek alphabet 'delta' for its commonly triangular shape by Greek historian Herodotus in the 5th century, deltas come in a variety of shapes and sizes.
The bengal delta lies on the eastern side of the Indian subcontinent and covers most of Bangladesh and west bengal of India. Ancient and modern deltaic plains cover about 65% of Bangladesh and the remaining 35% is alluvial plain and hilly terrain. The Bengal Delta is one of the most densely populated areas of the world. It occupies most of the bengal basin and is slowly subsiding as a result of isostatic adjustment of the crust due to the rise of the himalayas and dewatering of the Proto-Bengal-Fan sediments which are now buried under thick Mio-Pliocene-Pleistocene deltaic sediments.
The area south of a line drawn along the ganges and padma as far as the lower course of the feni river in the southeast belongs physiographically to the Delta of the Ganges, brahmaputra (jamuna) and meghna river. The Bengal Delta can be subdivided into moribund, immature, mature and active deltas.
The moribund delta areas are in the southwestern part of the country, south of the Ganges river, where tributaries are highly silted and are profusion of oxbow lakes. Such areas are Jessore, Kushtia, and Faridpur. The immature delta is on the south of the moribund delta and mainly consists of sea beach and tidally influenced lands. The sundarbans belongs to this region. subsidence is high in the immature delta. The mature delta part is more towards the central part of the southern section of Bangladesh (Patuakhali, Barguna, etc), where areas vary in their relief and tidal affects. The active delta is situated mainly at the estuary of the Meghna river entering the bay of bengal (Bhola, Hatiya, Sandwip, etc). Regular flooding and formation of chars and offshore islands are its characteristics. [Sifatul Quader Chowdhury]
See also bengal delta.