Abstract: Following the successful discharge of the Tangjiashan landslide-- dammed lake in Beichuan County, Sichuan Province, triggered by the 2008 Wenchuan Earthquake, a new water supply reservoir is planned at the site. The proposed dam location lies upstream of Dashuigou, on the left flank of the Tangjiashan landslide, where debris flow hazards pose a major threat to reservoir operations. This study employs field investigations, UAV photogrammetry, and SBAS-InSAR technology to identify potential debris flow sources, estimate storage volumes, and monitor deformation in the Dashuigou watershed. The results indicate the identification of 12 sources with distinct deformation characteristics, including earthquake-induced fractured mountain slopes, landslides, and slope debris. These sources are primarily distributed along the back scarp of the original Tangjiashan landslide, on both sides of the middle and upper reaches of the Dashuigou watershed, and along the downstream slopes. From January 2018 to October 2024, deformation rates of these sources in the watershed ranged from −120.28 mm/year to 48.41 mm/year, with maximum deformation depth reaching 18 meters at the back scarp of the Tangjiashan landslide and a total estimated volume of 38.73×10⁴ m³. The displacement of these sources shows a positive correlation with rainfall, with significant increases in deformation observed prior to three major debris flow events in Dashuigou in 2018, 2020, and 2021. The region’s steep topography, ongoing deformation of earthquake-induced fractured mountain sources, and high rainfall contribute to the occurrence of debris flow hazards, which pose a threat to the proposed dam site by potentially obstructing the Tongkou River. The findings of this study provide critical support for debris flow disaster prevention and mitigation efforts in the region.