On 07 February 2021 around 10:30 hrs local time catastrophic flash flood occurred in the Dhauliganga River (a tributary of Ganga River) near Rini village at 2000m above msl (mean sea level) (Chamoli District), which killed 79 people and about 125 people are still missing. Part of the area belongs to Nanda Devi Biosphere Reserve which is completely protected from human interventions. Further, on Dhauliganga River, there were two run-of-river hydroelectric power Rishiganga Small Hydro (13.2 MW) at 1975m above msl and Tapovan Vishnugad (520 MW) at 1795m above msl projects which were also severely damaged due to the devastating flash flood. More than 150 workers were also trapped in the under construction power tunnel of Tapovan Vishnugad project. Further, it is important to note that the study area belongs to highly seismically active region and Zone-V Seismic Zoning Map of India which comes under highly vulnerable category. During last thirty years the region has also witnessed about 53 earthquakes (within 100km) of magnitude ranging Mw 3.2–6.6. Similarly, landslide hazard zonation map of Uttarakhand also shows that the study area belongs to very high to severe category. Initial assessment on the day of event suggested that there was a glacial burst. However, later series of remote sensing images revealed that initially there was heavy snow fall during 03-05th February 2021 in the region. Later, it has been evaluated through time series high spatial resolution remote sensing images of various satellites that a large part of a north facing triangular shaped slope (having about 660m base and about 1100m height, estimated total volume 18.1 million m3 which includes rocks, snow and ice) at 5540m above msl has failed which was also supporting a small hanging glacier. This landslide and consequently massive debris flow into the Raunthi Gadhera initially blocked flow of Dhauliganga near Rini village [at 2000m above msl (mean sea level)] which later failed around 10:30 hrs on 07 February 2021 and has brought a catastrophic flash flood in the Dhauliganga river. Further, remote sensing images acquired around 10:33 hrs of 07th February 2021 revealed a large dust cloud which clearly unravels the sequence of events from high-altitude landslide, collapse of a small hanging glacier, snow avalanche to catastrophic flooding. Even after the catastrophic flash flood of 07th February 2021, an elongated lake (length about 800m, width at the front is about 100m and having depth of about 46m including freshly deposited debris and silt of about 10m, an estimated total volume of water might be ≈ 800000 m3) was created due to blocking of flow of Rishi Ganga River which still postures flash flood hazard for the downstream areas.