On 7 Feb 2021, a catastrophic mass flow descended the Ronti Gad, Rishiganga, and Dhauliganga valleys in Chamoli, Uttarakhand, India, causing widespread devastation and severely damaging two hydropower projects. Over 200 people were killed or are missing. Our analysis of satellite imagery, seismic records, numerical model results, and eyewitness videos reveals that ~27x106 m3 of rock and glacier ice collapsed from the steep north face of Ronti Peak. The rock and ice avalanche rapidly transformed into an extraordinarily large and mobile debris flow that transported boulders >20 m in diameter, and scoured the valley walls up to 220 m above the valley floor. The intersection of the hazard cascade with downvalley infrastructure resulted in a disaster, which highlights key questions about adequate monitoring and sustainable development in the Himalaya as well as other remote, high-mountain environments.

In 2017-2019 a surge of Shispare Glacier, a former tributary of the once larger Hasanabad Glacier (Hunza region), dammed the proglacial river of Muchuhar Glacier, which formed an ice-dammed lake and generated a small Glacial Lake Outburst Flood (GLOF). Surge movement produced the highest recorded Karakoram glacier surface flow rate using feature tracking (~18 ± 0.5 m d −1) and resulted in a glacier frontal advance of 1495 ± 47 m. The surge speed was less than reports of earlier Hasanabad advances during 1892/93 (9.3 km) and 1903 (9.7 km). Surges also occurred in 1973 and 2000-2001. Recent surges and lake evolution are examined using feature tracking in satellite images (1990-2019), DEM differencing (1973-2019), and thermal satellite data (2000-2019). The recent active phase of Shispare surge began in April 2018, showed two surface flow maxima in June 2018 and May 2019, and terminated following a GLOF on 22-23 June 2019. The surge likely had hydrological controls influenced in winter by compromised subglacial flow and low meltwater production. It terminated during summer probably because increased meltwater restored efficient channelized flow. We also identify considerable heterogeneity of movement, including spring/summer accelerations. Hasanabad was a surge-type glacier situated on the north flank of Hunza Valley in the Central Karakoram (Fig. 1). During most, but not all, of the 20th Century, the Shispare (or Shisper) and Muchuhar tributaries of the Hasanabad Glacier have been separated. Shispare Glacier recently gained attention of the media, scientific community, policy makers, and disaster response agencies when it surged, blocked the outlet stream of Muchuhar Glacier and formed a lake 1-5 (referred to hereafter as Shispare Lake), which then drained, and has recently begun reforming. Surge-type glaciers oscillate between brief (months to years) rapid flow and lengthy (tens to hundreds of years) slow flow or stagnation, which are called the 'active' (or 'surge') and 'quiescent' phases, respectively 6. In the former, a large volume of the glacier's mass is rapidly transferred from an upper 'reservoir zone' into the lower 'receiving zone. ' Conversely, in the quiescent phase, the lower glacier slowly stagnates, thins, and retreats, while the upper part builds mass towards another surge 6,7. Surging occurs in both temperate and polythermal glaciers 6-11. Earlier, two primary mechanisms were proposed to explain the change between surge and quiescent flow. (1) Thermally regulated surges generally switch from largely cold-based ice in quiescence, to warm-based ice during the surge (e.g., Monacobreen Glacier, Svalbard) 12. (2) Hydrological regulation (e.g., Variegated Glacier, Alaska) invokes switching from low water pressure in subglacial channels of temperate glaciers to high water pressure, usually with restructuring of subglacial drainage 8,11,13,14. However, more recently, a uniform model of surging has been proposed to integrate both processes under an enthalpy framework 7. The enthalpy balance theory explains...