Mangrove forest of Sunderban

The land is moulded by tidal action, resulting in a distinctive physiography. An intricate

network of interconnecting waterways, of which the larger channels of often a mile or more

in width run in a generally north-south direction, intersects the whole area. Innumerable small

khals drain the land at each ebb. Rivers tend to be long and straight, a consequence of the

strong tidal forces and the clay and silt deposits which resist erosion. Easily eroded sands

collect at the river mouths and form banks and chars, which are blown into dunes above the

high-water mark by the strong south-west monsoon. Finer silts are washed out into the Bay of

Bengal but, where they are protected from wave action, mud flats form in the lee of the

dunes. These become overlain with sand from the dunes, and develop into grassy middens.

This process of island building continues for as long as the area on the windward side is

exposed to wave action. With the formation of the next island further out, silt begins to

accumulate along the shore of the island and sand is blown or washed away (Seidensticker and

Hai, 1983). Apart from Baleswar River the waterways carry little freshwater as they are cut

off from the Ganges, the outflow of which has shifted from the Hooghly-Bhagirathi channels

in India progressively eastwards since the 17th century. They are kept open largely by the

diurnal tidal flow (Seidensticker and Hai, 1983).