Was Landslide On Sikkim Lake's Bank A Cause Of Deadly Floods? What Satellite Images Show

New high-resolution satellite imagery from Friday accessed by NDTV shows the exact site of the breach at the high-altitude South Lhonak Lake in Sikkim which resulted in disastrous floods, causing the loss of at least 50 lives.

Significantly, the images show exposed parts of the lake's banks, indicating that the water level continues to drop significantly after draining out of the breach, and causing massive floods along the Teesta River basin downstream.

There is also evidence of a landslide, which may been a contributing factor in the lake bursting its banks. 

"The glacier has significant snow presently, and this snowpack might have exerted tremendous pressure on the snout of the lake, which gave rise to the breach," said Arup R Dasgupta, Managing Editor of Geospatial World (formerly GIS Development) magazine, and a veteran ISRO imagery expert. 

The South Lhonak Lake is located at an altitude of 17,100 feet in the upper reaches of northern Sikkim, not too far from the India-China boundary. 

The new images show the exact area where there was a breach in the glacial lake.  One of the images indicates that water continued to flow out from the lake even on Friday, three days after it burst its banks. 

As a result of the decreased level of water in the lake, a large area of the shoreline is now exposed, an area which was under water just three days ago. 

In fact, Indian Space Research Organisation (ISRO) scientists have already stated that the area covered by the lake reduced by more than half, with only an estimated 60.3 hectares of water in it.

The South Lhonak Lake was also fed by glacial run-off from the retreating North Lhonak glacier and the main Lhonak glacier. This increased the lake's surface area by 500 metres and average depth by 50 metres, Dr SN Remya, the lead scientist of the 2013 paper, said.

Images from February this year show that the lake was entirely frozen, though a pattern of fractures on the surface of the ice is clearly visible. 

However, Friday's image shows a large volume of broken ice, and ice floes (sheets of floating ice), on the surface of the lake. It is not clear if the ice gave way because of the flow of underlying water which rushed to the breach or had melted substantially over the summer months. 

"The ice cover on the lake in the first images shows a regular fracture pattern, which indicates that the ice sheet was under pressure, possibly from the glacier. This pressure was perhaps increased due to fresh snow on the glacier as seen in the second image. This might have led to the breach," said Mr Dasgupta.

The image from October 6 also shows clear evidence of a landslide on one of the banks of South Lhonak Lake. It is unclear if the landside resulted in the displacement of water and was a contributory factor, which resulted in the lake breaching its banks.

"In Sikkim, lake-terminating glaciers have shown accelerated growth... South Lhonak glacier is no different. It is one of the fastest-retreating glaciers and the associated proglacial South Lhonak Lake has become the largest and fastest-growing in the state... this has raised concerns about hazard potentials as downstream (areas) are heavily populated..." a study published in the journal Geomorphology highlighted in 2021.