Context: Seven persons were killed and over 125 reported missing after a “glacial burst” on Nanda Devi triggered an avalanche and caused flash floods in Rishi Ganga and Dhauliganga rivers in Chamoli district of Uttarakhand.

Cause under study:

• A glacial lake burst, a cloud burst or an avalanche, the impact of climate change or “development” — scientists are not sure what triggered the sudden surge of water near Chamoli in Uttarakhand.
• The scenario being most talked about was what glaciologists like to call a GLOF, or glacial lake outburst flood. It is a reference to flooding caused downstream due to a breach in a glacial lake.
• Retreating glaciers, like several in the Himalayas, usually result in the formation of lakes at their tips, called proglacial lakes, often bound only by sediments and boulders.
• If the boundaries of these lakes are breached, it can lead to large amounts of water rushing down to nearby streams and rivers, gathering momentum on the way by picking up sediments, rocks and other material, and resulting in flooding downstream.
• GLOF events are not unusual, but their impact depends on the size of the proglacial lake that burst, and location.
• An avalanche is quite common, and there could have been one, but an avalanche on its own would not result in an increase in the flow of water in the river. The water has to come from a source.

Climate change impact:

• Environmental experts have attributed the Nanda Devi glacial melt to global warming.
• Glacier retreat and permafrost thaw are project-ed to decrease the stability of mountain slopes and in- crease glacier lakes.

Dhauliganga:

• The water level of Dhauli-ganga river at Joshimath flowed at a perilously high level, breaching all records.
• The Dhauliganga is one of the six source streams of the Ganges It meets the Alaknanda River at Vishnuprayag at the base of Joshimath mountain in Uttarakhand.

https://www.thehindu.com/todays-paper/7-killed-after-glacial-burst-in-uttarakhand/article33777839.ece

https://www.thehindu.com/todays-paper/experts-point-to-climate-change-impact/article33777851.ece

https://www.thehindu.com/todays-paper/dhauliganga-at-an-all-time-high/article33777904.ece

https://indianexpress.com/article/explained/uttarakhand-glacier-break-flash-flood-rishiganga-7179003/

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