Initial flash flood event of February 7, 2021 occurred in Raunthi Gadhera that originates from Nanda Ghungti (6309m) peak and flowing from south to north meeting with Rishiganga river (2315m asl) at right angle. Flooded water in a huge amount flowed down through the same that damming at the confluence of Raunthi Gadhera with Rishiganga river. Wherein back flowed of water saturated bouldary debris along the Rishignaga river upto 500m. After breaching of temporary dam at confluence, this left behind a huge amount of sediments over Rishiganga river upto at an elevation of 2389m asl and blocked the course of the same resulting in an artificial lake is formed. Flooded water gushing down along the Rishiganga washing away a functional hydropower project of 13.2 MW capacity on Rishiganga upstream of Rini and to the downstream severe damage under construction hydropower project of 520 MW of National Thermal Power Corporation (NTPC) at Tapoban on Dhauliganga river. Apart from these washing away 5 pedestrian bridges over Dhauliganga river along with 1 RCC motorable bridge over Rishiganga river. As many as 204 persons went missing, including 9 inhabitants and 2 Police personnel, of these body of 77 persons could be recovered. The study highlights causes of lake formation and suggestions for mitigating future threat from the same in downstream.