Depression3 relatively sunken parts of the earth's surface; especially a low-lying area surrounded by higher ground and having no natural outlet for surface drainage; usually deeply flooded during wet season. Tectonically, a depression is a structurally low area in the crust, produced by negative movements that depress, down sink or down thrust the rocks. Geologically, the beels, may be called basin areas which are usually not discernible as low lying, though they become obviously so during the rainy season and present a vast sheet of water. Depression type of landform may be a backswamp or abandoned old riverbed where runoff water accumulates to appear like a basin. Depressions of smaller sizes are amply seen associated with floodplains of Bangladesh.
A large number of depressions are found on the plains of Bangladesh and many of these are vast. The depressions are locally known as beels or haors. These are the result of natural physical phenomena but not all of those have the same origin. The mode of their origin has certain geological and hydrological aspects. Many of these have originated from fluvial processes and many others have originated due to earth subsidence. Examples of some of the major depressions in Bangladesh are Lower Atrai Basin, Lower Punarbhaba Floodplain, Arial Beel, Gopalganj-Khulna Beels (1,160 sq km), and Sylhet Basin (4,450 sq km) which is popularly known as haor area consisting of a number of fairly large depressions like Hakaluki Haor, tanguar haor, Sonir Haor etc.
The Sylhet Basin or surma basin is rapidly sinking showing in some places a subsidence of about 10.5m in the last few hundred years. Another estimate shows that the subsidence rate may exceed 20 mm/year in this basin. The Lower Atrai basin is popularly known as chalan beel. Part of this wetland has been brought under flood control projects through embankment. HYV (high yield variety) boro is cultivated in the area. The Lower Punarbhaba Floodplain is relatively small (129 sq km), located at the northwest edge of the barind tract (Varendra Bhumi in Bangla) and subject to flash floods, sometimes by early flooding by adjoining local runoff water. The Arial Beel is located between the Padma and Dhaleshwari floodplains.
Soils of these depressions are mainly dark grey, acidic, heavy clay. Either HYV boro or deepwater rice (floating aman) is grown here. Gopalganj-Khulna Beels are known as peat basin lying in the south central Ganges deltaic part, has risen only 30-60 cm above sea level. The depressions of Gopalganj-Khulna Beels are tectonic controlled, and still subsiding. The area remains deeply flooded in the monsoon and the southern part is saline. The areas are used for either deepwater aman or for local boro. There are many depressions also found in Brahmaputra-Jamuna Floodplain. peat, formed in the large depressions, is one of the major natural resources of Bangladesh. [Mohd Shamsul Alam]