2023 Sikkim flash floods

Satellite image from 2006 showing the expansion of the lake over four decades

The South Lhonak Lake is a moraine-dammed lake fed by the meltwater of the Lhonak glacier. It was first seen in CORONA satellite images from 1962 as a supraglacial lake. Landsat MSS images show that it became a separate lake by 1977, with a surface area of 17 hectares (42 acres). In four decades, as the glacier retreated 1.9 kilometres (1.2 mi), the lake swelled in size, covering nearly 100 hectares (247 acres) by 2008.[5] It was identified as potentially at risk of causing glacial outburst floods, and in 2018 pipelines were carried up by yak and installed to pump water out of it.[6] Sentinel-1A images from 28 September 2023 showed the lake covering an area of 167.4 hectares (414 acres).[7]

Before the flood, scientists and authorities were working on installing an early warning system for glacial floods from the lake.[8]

In early October 2023, a cloudburst caused Sikkim to receive more than double its normal rainfall;[9] between 3 and 4 October alone, the state received five times the usual precipitation.[2] The South Lhonak burst its shores, causing a flash flood. Satellite images from the Indian Space Research Organisation's RISAT-1A show that the lake's surface area shrunk by more than 100 hectares (247 acres). Based on a warning from the Indo-Tibetan Border Police at midnight, the operators of the Teesta III Dam at Chungthang rushed to open the dam's gates, but were too late; the flood quickly destroyed the dam, as well as the bridge to its 1200-MW hydroelectric powerhouse, which was submerged.[3]

Water levels downstream in the River Teesta subsequently rose by 15 to 20 feet (4.6 to 6.1 m), flooding many areas in Mangan, Gangtok, Pakyong, and Namchi districts in Sikkim, and Kalimpong, Cooch Behar, Jalpaiguri and Darjeeling districts in West Bengal.[10] The flood also went onwards to Bangladesh, where it affected hundreds of villages along the Teesta River and Char areas.[11]

Fifteen bridges across the state were washed away, and the north of the state, including the capital Gangtok, was cut off from the rest of India as parts of National Highway 10 collapsed.[2] Three thousand tourists were stranded across the state.[12] Towns and cities like Chungthang, Dikchu, Singtam, Rangpo, Melli, and Teesta Bazaar were very heavily damaged.