River

A river tends to eliminate irregularities and forms a smooth gradient from its source to its base level. As it approaches base level, downward cutting is replaced by lateral cutting, and the river widens its bed and valley and develops a sinuous course that forms exaggerated loops and bends called meander. A river can open up a new channel across the arc of a meander, thereby cutting off the arc and creating an oxbow lake. River velocity determines quantity and size of rock fragments and sediment carried by the river. Whenever velocity is checked by changes of flow of gradient, by meeting the water mass of lakes or ocean, or by the spreading of water when a stream overflows its banks, part of the load carried by the stream is deposited in the river bed or beyond the channel. Landforms produced by deposition include the delta, the floodplain, the channel bar, and the alluvial fan and cone.

Traditionally, river systems have been classified according to their stage of development as 'young', 'mature', or 'old'. The young river is marked by steepsided valley, steep gradients, and irregularities in the bed; the mature river by a valley with a wide floor and flaring sides, by advanced headward erosion by tributaries, and by a more smoothly gradient bed; and the old river by a course graded to base level and running through a peneplain, or broad flat area. Most of the rivers of Bangladesh are at their old stage and enter into the bay of bengal.

River valleys have been important centres of civilisation, they afford travel routes, and their alluvial soils form good agricultural lands. Navigable rivers are important in commerce and have influenced the location of cities. In Bangladesh, almost all the major cities/towns and commercial centres are located on the banks of rivers, eg, Dhaka on the bank of buriganga, Narayanganj by the side of shitalakshya, Chittagong by the side of karnafuli, and Mymensingh by the side of Brahmaputra. Rivers with sufficient water volume, velocity and gradient can be used to produce hydroelectric power. At kaptai the river Karnafuli of Chittagong region is being utilised for producing hydroelectricity.