Ganga-Brahmaputra delta

Also called: Bengal delta, Sundarbans delta · Topic: Structure and Physiography of India: Himalayas, Plains, Peninsula, Coasts and Islands · NCERT: Class 7, Ch 1 "Geographical Diversity of India"; Class 9, Ch 2 "Shaping of the Earth's Surface"; Class 11, Ch 2 "Structure and Physiography"; Class 11, Ch 5 "Natural Vegetation"

Meaning

The Ganga-Brahmaputra delta (also called the Bengal delta or Sundarbans delta) is the world's largest delta. It is a fan-shaped area of new land built where the Ganga, the Brahmaputra and the Meghna drop their silt before reaching the Bay of Bengal, in West Bengal (India) and Bangladesh. It matters because it holds the world's largest mangrove forest (the Sundarbans) and the single largest tiger population in the world, and because sea-level rise, cyclones and salty water threaten it [5].

Explanation

How the delta forms

• A delta is a fan-shaped area of land formed at a river mouth.
• Cause: the river reaches the sea and slows down.
• Process:
• The slow river can no longer carry its fine silt, so it drops it.
• Its channel splits into distributaries (branches that flow away from the main river, towards the sea).

• Each branch drops more silt at its mouth.

• Result: new land slowly builds out into the sea.

• Why this delta is so big:
• The Ganga and the Brahmaputra are Himalayan rivers that carry huge amounts of sediment.
• The sea floor at the head of the Bay of Bengal is shallow, so the silt piles up into land.
• This is the last stage of the Northern Plain. The zones from north to south are Shiwalik foothills → bhabar → tarai → bhangar/khadar → delta.

The rivers that build it

• Ganga: it splits at Farakka.
• The Hooghly (Bhagirathi-Hooghly) flows south through West Bengal.

• The main flow, the Padma, enters Bangladesh.

• Brahmaputra: it rises in Tibet as the Tsangpo, flows south-west through the Assam valley, and then turns almost 90° south at Dhubri (western Assam). It enters Bangladesh, where it is called the Jamuna.

• Meeting point: the Padma joins the Jamuna and then the Meghna. The joined river reaches the Bay of Bengal.
• The river landforms here belong to the mature stage (gentle slope; the river deposits sediment and flows sideways instead of cutting down). Examples are meanders, sand bars and many shifting channels.

The Sundarbans: the mangrove forest of the delta

• The Sundarbans is the world's largest delta and the world's largest mangrove forest. It is named after the sundari tree.
• Mangroves are salt-tolerant trees of tidal coasts. Many have breathing roots that stick up out of the mud. Tides flood the land twice a day, so the trees must survive in salty, wet mud.
• Area: about 10,000 sq km. About 60% lies in Bangladesh and the rest in India [5].

• UNESCO's split gives India about 40%. NCERT books differ: one says "about half" and another says "about one-third". In every version, India holds the smaller share.

• Mangrove cover in India:

• West Bengal had 2,119.16 sq km of mangrove cover in ISFR 2023. That is 22.16 sq km more than in ISFR 2013 (2,097 sq km) [2].

• West Bengal has India's largest mangrove cover, at 42.45% of the national total (ISFR 2023) [2].

• Wildlife: the Royal Bengal tiger lives here. It is the single largest tiger population in the world. These tigers have adapted to an almost amphibious life (living on land and swimming between islands) [5].

Conditions and threats

• Low and flat land: the delta sits barely above sea level, so the sea can easily flood it.
• Cyclones: the Bay of Bengal sends frequent cyclones and storm surges (walls of seawater pushed onto land by strong storm winds) into the delta.
• Salinity: less fresh water now flows down the rivers, so the water is getting saltier.
• Sea-level rise: a UNESCO report warns that it could destroy 75% of the forest by 2100 [5].

Where it occurs

• India: southern West Bengal, along the Hooghly (Bhagirathi-Hooghly) branch of the Ganga. Sundarbans National Park is here. It became a UNESCO World Heritage Site in 1987 [6].
• Bangladesh: the larger part (about 60%) is formed by the Padma, the Jamuna (Brahmaputra) and the Meghna [5].
• The Bangladesh Sundarbans is a separate World Heritage Site [5].

• It is also a Ramsar site (a wetland of international importance under the Ramsar Convention). It was designated in May 1992 and enlarged from 596,000 to 601,700 ha [7].

• Water body: the head of the Bay of Bengal.

• Waterway link: National Waterway-1 runs on the Ganga-Bhagirathi-Hooghly system, which flows through the Indian part of the delta.
• Other Indian deltas (for comparison): the Mahanadi, Godavari, Krishna and Kaveri build smaller deltas on the east coast. All of them drain into the Bay of Bengal.

Don't confuse with

• Estuary: a delta forms where a river drops its silt and builds land at its mouth (the Ganga-Brahmaputra). An estuary is a funnel-shaped mouth where tides wash the sediment away, so no delta forms. The west-flowing Narmada and Tapi form estuaries.
• Riverine island (Majuli): Majuli is land inside the braided channel of the Brahmaputra in Assam, far upstream. It is not part of the delta. It is the "world's largest inhabited river island", and it is shrinking because of erosion [3][4].
• Khadar: khadar is new alluvium on river floodplains that floods renew almost every year. The delta is the land a river builds at its mouth by splitting into distributaries. Both are fertile and made of fine silt, but they sit in different places.
• Sundarbans National Park vs Bangladesh Sundarbans: these are two separate World Heritage Sites. India's was listed in 1987 [6]. Only Bangladesh's part is the Ramsar site designated in May 1992 [7].

Prelims Hooks

• World's largest delta and world's largest mangrove forest: the Sundarbans. About 60% of it lies in Bangladesh, not India [5]. This is a common "which of the following" trap.
• River order to the sea: the Ganga splits at Farakka → the Hooghly goes to West Bengal and the Padma goes to Bangladesh → the Padma joins the Jamuna (Brahmaputra) → then the Meghna → Bay of Bengal.
• The Brahmaputra turns south at Dhubri (Assam) and is called the Jamuna in Bangladesh.
• West Bengal has India's largest mangrove cover: 2,119.16 sq km, or 42.45% of the national total (ISFR 2023) [2].
• Protected status: India's Sundarbans National Park became a World Heritage Site in 1987 [6]. Bangladesh's Sundarbans became a Ramsar site in May 1992 [7].
• Name match: Sundarbans comes from the sundari tree. Its tigers are the Royal Bengal tigers, the single largest tiger population in the world [5].

Mains Points

• Climate threat to a low delta (GS-III, environment and disasters):
• Sea-level rise could destroy 75% of the Sundarbans forest by 2100 [5].
• Frequent Bay of Bengal cyclones and storm surges hit this flat, low land.

• Less fresh water from the rivers raises salinity, which harms the mangroves, the tigers and the people who live by fishing and farming.

• Mangroves as a natural shield (GS-III, disaster risk reduction):

• Mangrove roots slow down storm surges and hold the mud in place.
• So the gain of 22.16 sq km in West Bengal's mangrove cover between ISFR 2013 and ISFR 2023 matters, even though it is small [2].

• Planting and protecting mangroves is a low-cost defence for coastal villages.

• One delta, two countries (GS-I and GS-III):

• The delta is shared, and Bangladesh holds about 60% of it [5]. Its two parts have separate World Heritage listings [5][6].
• The rivers, the silt, the tigers and the cyclones all cross the border. So India and Bangladesh need to work together on river flows, fresh-water supply and conservation.

Related concepts

• Indo-Ganga-Brahmaputra plain
• Bhabar
• Tarai
• Bhangar
• Khadar
• Piedmont alluvial plains
• Riverine island
• Majuli

Read more

• The Northern Plain: bhabar, tarai, bhangar, khadar and the deltas