Bangladesh's geography: a delta built by rivers from somewhere else

How three rivers built most of the country

The delta plain of the Ganges, the Brahmaputra and the Meghna covers 79% of Bangladesh by the measure in the Wikipedia geography article, which puts the four uplifted blocks of old alluvium at 9% and the steep hill ranges of the east at 12%. Most of the plain is under 10 metres above sea level, falling to sea level along the southern coast, and Britannica describes the recent alluvium as a flat surface with a gentle slope that is generally below 9 metres. About 10,000 km² of the country is permanent water, and much more floods each monsoon.

The material came from upstream. The Ganges and the Brahmaputra deliver nearly 1,000 million tonnes of sediment a year, which has built both the Bengal Delta and a submarine fan extending south of the equator, up to 16.5 km thick. Britannica puts the greater Ganges Delta at about 60,000 km², most of it in south-western Bangladesh.

The land is still being made. A study of the coastal zone summarised in the geography article found 28,835 km² of land in 1989 and 29,426 km² in 2018, a net gain of 590 km² over thirty years, or about 20 km² a year, most of it new islands at the mouth of the Meghna while other islands lost ground. New land starts as mud and turns to grass and then trees within those three decades.

Bangladesh measures 767 km from north to south and 454 km from east to west. The soils follow the age of the deposits: the recent alluvium of the floodplains is fertile clay and loam, short of phosphoric acid, nitrogen and humus but not of potash or lime; the older alluvium of the Barind and Madhupur tracts is a sticky red-brown clay; the hill soils are permeable and carry forest.

Where the rivers come from and who controls them

Britannica states it without qualification: none of the major rivers of Bangladesh originates within its territory. The Padma rises in Nepal as the Ganges, the Jamuna in China as the Yarlung Tsangpo, and the Surma in India, and all three reach Bangladesh across Indian land. The country has about 700 rivers, and almost all of them flow south.

The four connected systems drain about 1.5 million km², most of it outside the country. Their combined flow runs to about 140,000 m³ a second in the monsoon and falls to about 7,000 m³ a second in the dry season, a twentyfold swing.

The dry-season flow is the one that has been negotiated. India built a barrage at Farakka in West Bengal, upstream of the border, which diverts Ganges water within India, and Britannica records that the flow reaching western Bangladesh has since been insufficient from November to April. Bangladesh took the dispute to the United Nations in 1976, reached a bilateral agreement in 1977, and signed a Ganges water-sharing treaty with India under the government of 1996 to 2001. Britannica describes the division of the river's water as a source of friction between the two since the 1970s. Salinity in the Sundarbans has moved farther inland because river water is diverted upstream.

Why the Jamuna runs where it does

The main course of the Brahmaputra through Bangladesh is about two centuries old. In 1787 the Tista flooded exceptionally high and its waters swung east into the Brahmaputra, which began cutting into a minor stream; by the early 1800s that stream had become the river's main lower course, now called the Jamuna. The former bed carries a much smaller river, the Old Brahmaputra. Britannica concludes that no description of the country's topography remains accurate for long.

The Jamuna shifts subchannels constantly and throws up chars, silt islands fertile enough to be farmed and unstable enough to vanish. No permanent settlement stands along its banks, and its tributaries, entering mostly from the west, hamper links between the north of the country and Dhaka. The upper Padma changes its main channel every two or three years except where high banks confine it, and Britannica records that its silt islands have for decades been a source of feuds between farmers racing to occupy them.

Rivers are also the thoroughfares. The larger ones are the main source of irrigation water and the principal routes for commercial transport, and they supply fish, a main source of protein.

How the monsoon sets the agricultural year

The Bangladesh Meteorological Department recognises six seasons, and the climate turns on one of them. About 80% of the rain falls in the monsoon, from June to September, when moist air from the Indian Ocean travels up the Bay of Bengal and is turned west by the Himalayan foothills. Rainfall in most of the country is at least 2,300 mm a year. The dry west around Rajshahi receives about 1,600 mm, and the Sylhet region in the north-east, directly under the turn, recorded between 3,280 and 4,780 mm a year from 1977 to 1986.

The north of the country, above Dhaka, has a humid subtropical climate and the south a tropical monsoon climate. April is the hottest month, with maximum temperatures of 38 to 41 °C, and January the coolest, averaging 16 to 20 °C by day and about 10 °C at night. Violent thunderstorms from March to May, called northwesters, bring winds of up to 60 km an hour.

Farming is organised around the two regimes. More than 60% of net arable land, some 91,000 km², is cultivated in the rainy season despite the flood risk, and nearly 40% in the dry winter, which depends on irrigation from tube wells and pumps. Rice is grown in three seasons. Arable land made up 58.96% of the country in 2012, and the rivers flood their banks each year between June and October, peaking in September or October and receding quickly in November.

What floods and cyclones have cost

Britannica counts more than 1,000,000 people killed in storms since records began in the early eighteenth century, some 815,000 of them in three storms, in 1737, 1876 and 1970. Storms strike early in summer, in April and May, and late in the monsoon, from September to November, and can raise waves of 6 metres in the Bay of Bengal before they break on the coast and the islands.

The losses of the late twentieth century are recorded in detail. The Bhola cyclone of November 1970 killed an estimated 300,000 to 500,000. A cyclone in May 1985 killed more than 11,000 people and some 135,000 head of livestock. In 1988 floods damaged two-thirds of the country's districts, covered half of Dhaka including the airport runway, and destroyed about 2 million tonnes of crops. A cyclone in 1991 killed some 130,000, and the monsoon flood of 1998 covered about two-thirds of the territory for two months and left more than 30 million people homeless.

The response has been warnings and shelters. The geography article's summary of the 1980s is that no protection against a cyclone or tidal bore existed except advance warning and public buildings to shelter in. The World Bank, in an overview updated in April 2025, credits Bangladesh with reducing cyclone deaths a hundredfold since 1970, and reports that $1.4 billion of its financing has built shelters, roads and embankments for about 6.7 million people. Britannica adds the longer risk: a rising sea threatens large parts of the coastal delta, and the melting of Himalayan glaciers is expected to worsen flooding and erosion.

Water carries a hazard of its own below ground. Groundwater used for drinking and irrigation is contaminated with naturally occurring arsenic in parts of the floodplain.

Where the land rises above the plain

The exceptions to the plain are few and sit at the edges. The Barind in the north-west and the Madhupur Tract in the centre are blocks of older alluvium, rising to about 30 metres, with hillocks of 9 to 18 metres on the Madhupur plateau and stands of sal, a hardwood Britannica compares with teak. The Sylhet Hills in the north-east rise from about 30 to more than 330 metres.

The Chittagong Hill Tracts in the south-east are the one true hill system, the western fringe of the north-south ranges of Myanmar and India. Ridges of soft clay and shale, rarely wider than 36 metres at the crest, run north to south with fertile valleys between them. The sources disagree on their height: Britannica puts the ranges generally below 600 metres, the Wikipedia geography article at 600 to 900 metres. The record for this site names Mowdok Taung as the high point at 1,054 metres; the Wikipedia geography article gives the same summit at 1,052 metres and rejects two peaks that were once reported higher, Keokradong and Tazing Dong. The Karnaphuli, the one large river system unconnected to the others, cuts west through these hills and is dammed at Kaptai to generate hydroelectric power.

Forest covered about 14% of the land in 2020, 1,883,400 hectares, down from 1,920,330 hectares in 1990, and about 33% of the forest area lay within protected areas.

What the Sundarbans are and who shares them

The Sundarbans are a tract of mangrove forest and saltwater swamp at the seaward edge of the delta, about 260 km from the Hugli estuary in India to the Meghna estuary, and about 10,000 km² of land and water in all. UNESCO and Britannica agree that about three-fifths lies in Bangladesh. UNESCO describes the Bangladeshi reserve as a mosaic of islands washed by brackish water between the Baleswar and Harinbanga rivers; land and exposed sandbars make up 70% of it and water 30%. The name is thought to come from the sundari tree, the most plentiful of the mangroves.

Three wildlife sanctuaries in the Bangladeshi part, covering 139,700 hectares, were inscribed as a World Heritage site in 1997, adjoining the Indian site inscribed in 1987. UNESCO's statement lists the Royal Bengal tiger, Ganges and Irrawaddy dolphins, estuarine crocodiles and the river terrapin, alongside 334 plant species and 315 species of bird. It names cyclones, saline intrusion, siltation, timber cutting, poaching and farm encroachment as the threats.

To the east, the coastal plain runs south from the Feni River to Cox's Bazar, 1.6 to 16 km wide, partly sand and partly saline clay. Offshore lie several islands, among them St. Martin's, which Britannica identifies as the country's one coral reef, off the coast of Myanmar.

Why the official area and borders vary by source

Bangladesh gives its area as 147,570 km², the figure of the Bangladesh Bureau of Statistics in 2020 and the one in the record. The CIA World Factbook gave 148,460 km² in 2021, of which 130,170 km² was land and 18,290 km² water.

The borders read the same way. The Wikipedia geography article gives a land frontier with India of 4,095 km and with Myanmar of 193 km in its own text, and quotes the Factbook's 4,156 km and 271 km in its table. The coastline is about 580 km, highly irregular and fissured by the mouths of many rivers. The territorial sea extends 12 nautical miles and the exclusive economic zone 200.