Ancient lake outburst ‘holds clues to climate change events’

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[CHENNAI] A glacial lake outburst that occurred in the Himalayas thousands of years ago holds clues to the dangers faced by the entire Indus river valley system from similar events triggered by rapid climate change, say scientists.

A team of Indian geologists from the University of Pune and the Wadia Institute of Himalayan Geology, Dehradun, has been studying the site of the ancient outburst in a deep gorge close to the exit of the Indus river in the Spituk-Leh valley. 

The valley suffered an unprecedented cloud burst on last year (6 August) causing flash floods that devastated large areas and killed 180 people.

Sliding and slumping of rocks and lake beds, compounded by climate change can result in the formation of steep valleys, lake bursts and ‘damming’ or containment of water within narrow gorges.

Unlike other sediment records in the area, which have eroded, the study site on the outskirts of Leh is well preserved and displays sediment from the flooding of the Indus river in the Pleistocene age, at least 20,000 years ago.

The team reported this month (10 June) in Current Science that the present record of an entire lake burst at a height of 3,245 metres in Leh "is a witness to the susceptibility of the Indus river to damming (and outburst) due to distinctive geomorphic (earth) changes available at the present outlet of the Spituk-Leh valley".

Glacial recession, the possibility of new lake basins being created by glacial melt and the damming of rivers followed by lake outbursts and related flash floods are likely to increase, they said.

"The studied past record proves that the geomorphic setup (for lake outbursts) is available in the Leh valley for such a thing to happen when climate change of that dimension (most probably during late Pleistocene-Holocene times or around 10,000 years back) occurs (again)," lead scientist Satish Sangode told SciDev.Net.

"Extreme events, like cloud burst, excessive melting of glaciers and excessive rain definitely affect the valleys instantaneously," said Sangode, a professor in the department of geology at the University of Pune. 

Rivers such as the Indus run past narrow gorges and valleys, gathering sediment, while gullies and the hanging valleys in the mountains form natural dams. "Breaching of these dams can cause turbulent hydrologic conditions in the river," Sangode said.

"When the narrow valleys enter a broader open valley like Leh, the sediment-enriched water is suddenly released, producing flash floods," Sangode said.

Traditionally, local people have preferred settling in the broader open valleys. Of late, however, people have begun settling in areas that are less safe in an unplanned manner, Sangode said. 

Unplanned growth in the Himalayan region makes populations vulnerable to the impacts of events such as the 2010 cloud burst in Leh, Sangode said.