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. 

Uttarakhand disaster: a Glacial lake burst or avalanche

A GLACIAL lake burst, a cloud burst or an avalanche, the impact of climate change or “development” — scientists are not sure what triggered last year the sudden surge of water near Chamoli in Uttarakhand that briefly raised fears of a repeat of the 2013 disaster in the state.

• The scenario being most talked about was what glaciologists like to call a glacial lake outburst flood (GLOF).

• It is a reference to flooding caused downstream due to a breach in a glacial lake.

Glacial lake burst or avalanche? 

• But while GLOF is being considered to be the most likely trigger for recent event, there are questions surrounding this possibility. “We don’t know of any big glacial lakes in this region.
• 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.

What is the avalanche?

An avalanche, also called a snowslide, is an event that occurs when a cohesive slab of snow lying upon a weaker layer of snow fractures and slides down a steep slope. 

• Avalanches are typically triggered in a starting zone from a mechanical failure in the snowpack (slab avalanche) when the forces of the snow exceed its strength but sometimes only with gradual widening (loose snow avalanche).
• Falling masses of snow and ice, avalanches pose a threat to anyone on snowy mountainsides. Beautiful to witness from afar, they can be deadly because of their intensity and seeming unpredictability.

What is the Cloudburst?

Cloudburst is a sudden, very heavy rainfall, usually local in nature and of brief duration. Most so-called cloudbursts occur in connection with thunderstorms.

• A large amount of water may thus accumulate at high levels, and if the upward currents are weakened the whole of this water falls at one time.
• Cloudbursts are especially common in mountainous areas. 
• This is probably because the warm air currents of a thunderstorm tend to follow the upward slope of a mountain. 

The recent collapse in Uttrakhand:

The Himalayan rivers pass through environmentally fragile areas. However, like other Himalayan rivers, the Dhauliganga too has been dammed. 

The recent collapse of the glacier led to rippling flash floods in the Dhauliganga and the Rishiganga and also impacted the Alaknanda.

• A portion of the Nanda Devi glacier broke off in Uttarakhand’s Chamoli district on February 7.
• It triggered an avalanche and a deluge in the Alaknanda river system that washed away hydroelectric stations and trapped more than 100 labourers who are feared dead.
• The sudden flood in the middle of the day in the Dhauli Ganga, Rishi Ganga and Alaknanda rivers — all intricately linked tributaries of the Ganga — triggered widespread panic and large-scale devastation in the high mountain areas.
• Two power projects — NTPC’s Tapovan-Vishnugad hydel project and the Rishi Ganga Hydel Project — were extensively damaged.