Kosi river

Geography glossary

Also called: Sorrow of Bihar, Sapt Kosi · Topic: Drainage System of India: Rivers, Basins and Patterns · NCERT: Class 11, Ch 3 "Drainage System"

Meaning

The Kosi is a Himalayan left-bank tributary of the Ganga whose main stream, the Arun, rises north of Mount Everest in Tibet. In Nepal it is joined by the Son Kosi and the Tamur Kosi to form the Sapt Kosi, and it then flows into Bihar to join the Ganga.

It is called the "sorrow of Bihar" because of its floods and frequent changes of course. It is also the standard Indian example of an antecedent river, so it is a common Prelims and Mains topic.

Explanation

Why it is an antecedent river

  • An antecedent river is a river that is older than the mountains it crosses.
  • Cause: the river was already flowing before the Himalaya rose.
  • Process:
  • The land slowly rose under the river.

  • The river kept cutting down into the rock at the same speed as the land rose.

  • Result: a deep gorge (a narrow, deep valley with steep sides) cut right across the range.

  • The Arun, the Kosi's main stream, is a classic example. It starts on the Tibetan side, north of Everest, and still cuts through the main Himalaya.

Parts of the river: how the Sapt Kosi forms

  • Main stream: the Arun, which rises north of Mount Everest in Tibet.
  • After crossing the Central Himalaya in Nepal, the Arun is joined by:
  • the Son Kosi from the west

  • the Tamur Kosi from the east

  • Together they form the Sapt Kosi. The name means "seven Kosis", because many streams join to make it.

  • The Sapt Kosi then enters the plains of Bihar and joins the Ganga as a left-bank (north-side) tributary.

Nature of its flow

  • It is a snow-fed Himalayan river. Glaciers and snow feed it, and so does monsoon rain.
  • So it is perennial (it flows all year).
  • Like other left-bank Himalayan rivers, it cuts deep gorges in the mountains and carries a very large silt load.

Why it floods and shifts course

  • Heavy silt from young mountains:
  • The Himalaya are young mountains. Their rocks wear away easily.

  • So the Kosi picks up a huge load of silt in its upper course.

  • Silt settles in the plain:

  • When the river reaches the flat Bihar plain, it slows down.

  • A slow river cannot carry its load, so the silt settles on the riverbed.

  • Riverbed rises, water spills out:

  • The raised bed blocks the old channel and leaves less room for water.
  • In the monsoon the river spills over its banks and floods the land.

  • It often cuts a new channel and moves away from the old one. This is called a shift of course.

  • Result: repeated floods and lost farmland in north Bihar. This is why it is called the "sorrow of Bihar". Among the left-bank rivers, the Kosi is the worst case.

Where it occurs

  • Tibet (China): source of the Arun, north of Mount Everest.
  • Nepal: the Arun crosses the Central Himalaya. The Son Kosi (west) and Tamur Kosi (east) join it, and the Sapt Kosi forms.
  • India, Bihar: the river flows across the north Bihar plain and joins the Ganga in Bihar.
  • Place in the Ganga system (west to east): Ramganga → Ghaghara (with Sarda) → Gandak → Kosi → Mahananda. The Kosi lies east of the Gandak and west of the Mahananda, which is the last left-bank tributary and joins the Ganga in West Bengal.
  • Other antecedent rivers in India: the Indus, Sutlej and Brahmaputra.

Don't confuse with

  • Damodar ("sorrow of Bengal"): a right-bank Peninsular river that flows in a rift valley on the Chotanagpur plateau and joins the Hugli. The Kosi is a left-bank Himalayan river and is the "sorrow of Bihar".
  • Gandak: also formed in Nepal, from the Kaligandak and Trishulganga, and also joins the Ganga in Bihar, but at Sonpur, near Patna. The Kosi is formed by the Arun, Son Kosi and Tamur Kosi, and it lies further east.
  • Son river vs Son Kosi: the Son is a right-bank Peninsular river that rises at Amarkantak and joins the Ganga near Arrah. The Son Kosi is a Himalayan stream in Nepal that joins the Arun from the west.
  • Consequent river: a river that formed after the land took shape and simply follows its slope. An antecedent river like the Kosi (Arun) is older than the mountains and cuts across them.

Prelims Hooks

  • Kosi = sorrow of Bihar; Damodar = sorrow of Bengal. This pairing is a frequent trap.
  • Main stream of the Kosi: the Arun, which rises north of Mount Everest in Tibet.
  • Formation of the Sapt Kosi: Arun + Son Kosi (from the west) + Tamur Kosi (from the east).
  • Antecedent rivers of India: Indus, Sutlej, Brahmaputra and the Kosi (Arun). All are older than the Himalaya they cut across.
  • Left-bank Ganga tributaries, west to east: Ramganga → Ghaghara → Gandak → Kosi → Mahananda.
  • Process–problem pair: heavy silt load → raised riverbed → floods and shifting course. The Kosi is the textbook example.

Mains Points

  • Causes of the Kosi flood problem (GS-I / GS-III, disasters):
  • Young, easily eroded Himalaya → very heavy silt load.
  • Silt settles on the flat Bihar plain → riverbed rises.

  • The river spills over and shifts course → repeated floods and loss of farmland in north Bihar.

  • Policy responses (GS-III):

  • Embankments along the river to hold the water in its channel.

  • The river rises outside India, so the basin has to be managed jointly with Nepal. India and Nepal already have river-basin agreements, such as the Mahakali Treaty, 1996, which can serve as a model [2].

  • Geomorphic significance (GS-I): the Arun shows how an antecedent river keeps its course across a rising mountain range by cutting a deep gorge. This is key evidence that Himalayan drainage is older than the mountains themselves.

Related concepts

Read more

Sources

  1. 1Class 11, Ch 3 "Drainage System" (primary)
  2. 2Mahakali Treaty 1996 (Ministry of Jal Shakti)jalshakti-dowr.gov.in · tier 1