Due to the numerous and varied processes in the catchment, flash floods can cause significant harm to societies. Globally, it is evident that extreme rainfall events have increased in recent years due to climate change, and flash flooding is quickly becoming one of the most common types of natural disasters worldwide. Managing flash flooding is extremely challenging due to its suddenness, complex patterns, and difficulty in forecasting. According to popular belief, flash floods are highly concentrated, short-lived phenomena that peak at a high altitude less than six hours after rainfall. Nonetheless, these floods significantly affected most of the downstream portion of the river, where the valley gradient is minimal and is dramatically impacted by floods. The year 2021 witnessed a significant flash flood event that led to substantial geomorphic alterations in the upper watershed of the Teesta Basin, particularly in the South Lhonak Lake region in Sikkim, India. This event, due to its scale and impact, serves as a crucial case study for understanding the geomorphic changes caused by flash floods. This study will primarily examine the geomorphic modifications caused by the flash flood in the upper catchment area, focusing on the amount of precipitation, sediment dynamics, and landform deformation. It will also analyse the pre- and post-flood scenarios of the basin using data from remote sensing and GIS-based analysis. The probability of glacial lake outburst floods (GLOFs) has increased due to the rapid expansion of the proglacial South Lhonak Lake caused by accelerated glacial melting in recent years. Understanding the process is essential, as extreme events present additional challenges for risk management. This study will also emphasize the assessment of land use and land cover change after the flood scenario and analyze the peak discharge during the flood event using the hypsometric curve. Evaluate hazard management and minimize damage caused by extreme flood events. Therefore, to comprehend exposure, people’s socioeconomic vulnerability, and their ability to respond to floods, a flood risk assessment is required. The findings indicated that the midstream sections and upstream portions of the right bank of the Teesta River posed the greatest danger of flooding. There were notable differences in the level of flood risk among the upper watershed of the Teesta basin. A noteworthy inverse relationship was observed between vulnerability and capacity. The primary objectives of flood risk assessment are to enhance preparedness for potential future flood hazards, reduce flood risk through mitigation activities, and to compare pre- and post-flood scenarios.