Why In News?
Authorities placed the Sankosh River system on red alert following a Glacial Lake Outburst Flood (GLOF) in neighbouring Bhutan.
About Sankosh River
The Sankosh River (known as Puna Tsang Chu or Punatsangchhu in Bhutan) is a transboundary river in the Eastern Himalayas and a key right-bank (north-bank) tributary of the Brahmaputra River.
● Origin: It originates in northern Bhutan from glacial melt and snow in the Himalayas. It is formed by the confluence of two primary headstreams—the Pho Chhu and the Mo Chhu—just below the historic Punakha Dzong.
● Course in Bhutan: It flows for about 214 km through Bhutan, passing near Wangdue Phodrang (where it is joined by the Tang Chuu), generally moving through narrow gorges and steep gradients.
● Course in India: It crosses into India, traveling roughly 107 km. In its lower course, it forms a natural interstate boundary between Assam and West Bengal before emptying into the Brahmaputra River at Gauripur in Assam's Dhubri district. In the plains of India, it is also locally called the Gangadhar River.
● Total Length: 321 km (214 km in Bhutan + 107 km in India).
● Major Tributaries: Mo Chhu, Pho Chhu, Tang Chuu, Hara Chhu, and Daga Chhu.
About Glacial Lake Outburst Flood (GLOF)
A Glacial Lake Outburst Flood (GLOF) is a sudden and catastrophic flood caused by the breach of a natural dam (made of ice, rock, or moraine debris) holding back a glacial lake.
How Glacial Lakes Form
● As glaciers melt and retreat due to rising temperatures, they leave behind depressions or basins that fill with meltwater.
● Loose rocks, soil, and ice debris pushed by glaciers form natural walls called moraines, creating moraine-dammed lakes. These natural dams are structurally weak and unstable.
Causes / Triggers of GLOFs
● Climate Change & Rapid Melting: Rising global temperatures accelerate glacier retreat and expand the volume of water in glacial lakes.
● Avalanches & Landmass Displacement: Icefalls, rockfalls, or avalanches crashing into the lake create large displacement waves that overtop or fracture the weak dam.
● Seismic Activity: Earthquakes in fragile high-mountain zones (such as the Hindu Kush-Himalayas) can easily collapse natural dams.
● Internal Erosion (Piping): Seepage of water through loose moraine material progressively weakens the structural integrity of the barrier from within.
Impacts
● Flash Floods & Debris Flows: GLOFs unleash massive surges of water mixed with boulders, mud, and ice that travel rapidly down mountain valleys.
● Infrastructure & Human Loss: They devastate downstream settlements, bridges, roads, and critical hydropower projects.
● Notable Examples in India:
● South Lhonak Lake (Sikkim, 2023): Triggered severe flooding along the Teesta River basin and damaged the Teesta III Dam.
● Kedarnath / Chorabari Lake (Uttarakhand, 2013): A disaster involving cloudbursts and glacial lake drainage.
Source: THEHINDU
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PRACTICE QUESTION Q. With reference to the Sankosh River, consider the following statements:
Which of the statements given above are correct? (a) 1 and 2 only (b) 2 and 3 only (c) 1 and 3 only (d) 1, 2, and 3 Answer: (a) Explanation: Statement 1 is correct: The Sankosh River originates in northern Bhutan, where it is formed by the confluence of two primary headstreams: the Pho Chhu and the Mo Chhu near Punakha. Statement 2 is correct: In Bhutan, the river is known as the Puna Tsang Chu (or Punatsang Chhu). Statement 3 is incorrect: In India, the Sankosh River forms a natural inter-state boundary between Assam and West Bengal (not Meghalaya) before joining the Brahmaputra River. |
