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Landslide failures near water can generate impulse waves that can cause fatalities and property damage. These hazards are extremely prevalent in reservoir areas, such as the 1963 Vajont landslide in Italy and the 2003 Qiangjiangping landslide in China. This study introduces the Gapa landslide which has been reactivated by the impoundment of the Jinping reservoir, Southwest China. According to the geological investigation, the movement of the landslide was based on a deep sliding surface with a displaced body of 13 million m3. However, the landslide did not receive much attention until it exhibited huge displacements and extensive cracking in the upper part of the landslide, which was caused by the rapid drawdown of the reservoir water after rising to its highest level (1880 m asl). At present, the landslide has posed significant threats both to the Wali village on the opposite river bank and the downstream dam site during the subsequent reservoir water fluctuations. Therefore, the landslide's deformation behaviour and mechanism, and its failure mode need to be analyzed, Also, the assessment of the landslide-generated impulse waves is of great necessity. The displacement history of the landslide is presented using surface and underground monitoring data pre-, in-impoundment and subsequent five times fluctuations of the reservoir water. The analysis of daily displacement velocity and inter-annual displacement interprets the comprehensive spatio-temporal displacement characteristics between 2007 and 2020. Based on the interpretation of displacement behaviours, the deformation mechanism is revealed mainly using the hydrostatic and hydrodynamic forces. Additionally, the potential impulse waves have been predicted based on the inferred failure mode of the Gapa landslide. The maximum wave height on the river and the run-up wave height on the opposite bank are estimated to be 45.5 m and 35 m using the empirical and numerical methods. The Wali village below 1915 m asl will be destroyed in a hundred seconds if the landslide collapses overall, thus, a permanent warning line at 1915 m asl is required and residents below the line need to be relocated. The assessment of potential impulse waves related to the Gapa landslide can serve the risk management of the reservoir bank and the hydropower station. We anticipate that this study will provide some important insights into the interpretation of landslide hazard risk in the Jinping Reservoir area and other parts of the world. • The large scale and fully developed Gapa landslide with great threats are introduced. • The valuable monitoring data for 14 years from underground, surface and satellite-based is presented. • The spatiotemporal displacement behaviour is interpreted and the deformation mechanism is revealed. • The impact of landslide-generated impulse waves on the opposite bank and river chancel has been evaluated based on the inferred failure mode. [ABSTRACT FROM AUTHOR]