Himalayan region is highly susceptible to natural hazards particularly those that are triggered by the action of water. Due to the vast topographical diversity, events of ‘peak runoff’ pose various risks to small villages located at the watershed’s foot area. In this study, for the purpose to estimate flash flood risk along the Gagas River in Kumaun lesser Himalaya, high-resolution Digital Elevation Model (DEM) coupled with Geographical Information Systems (GIS) were utilised. The region experiences frequent storm events especially in the monsoon season. The river basin is also an evolving HELP basin endorsed by UNESCO as part of its global efforts for restoration of languishing river systems. Variability in the climatic conditions has imposed undue pressure on the livelihoods for survival. Relevant morphometric, topographic parameters and maximum runoff of the sub-watersheds of Gagas river watershed were computed in the GIS environment and were analysed to understand the drainage basins susceptibility to the flash flood hazards. These measurements allowed prioritising the sub watersheds in the presence of a series of rainstorms that generate unusual runoff volumes. Map representing hazard zones of sub-watersheds were identified and classified into four susceptibility groups (very high, high, moderate and low). The knowledge of flash flood susceptibility is important in mitigating the losses incurred to agriculture, irrigation systems, watermills, and recreational activities; and in the proper management of water resources.