[Objectives]Baihetan Hydropower Station, the key cascade project in the lower Jinsha River, plays a vital role in basin water regulation and power generation. Reservoir sedimentation seriously impairs reservoir storage capacity, operational safety, and power generation benefits. Complicated bank geological conditions lead to frequent debris flow activities and uncertain interval sediment supply, greatly affecting the sediment erosion-deposition evolution of the reservoir. To address the above issues, this study aims to analyze the spatial and temporal distribution characteristics of sediment deposition in the Baihetan Reservoir in 2025, quantify the sediment occupation of dead storage and regulating storage, and clarify the impacts of mainstream, tributary, and debris flow sediment transport on local deposition. The findings are expected to provide technical references for the reservoir's sediment management and long-term safe operation. [Methods] Based on the long-term field observation section data of the mainstream and tributaries in the Baihetan Reservoir, this study adopted the typical cross-section terrain method, which is widely applied in reservoir sediment calculation. Combined with the annual rainfall data of the Jinsha River basin and remote sensing image data of debris flow gullies in the reservoir area, the sediment erosion and deposition volume of the entire reservoir and each typical section in 2025 was calculated. The spatial distribution differences of sediment deposition between the mainstream and tributary estuaries were quantitatively analyzed, and the occupation characteristics of dead storage and regulating storage by sediment were statistically summarized. Meanwhile, the contribution of single debris flow gully sediment transport to local reservoir deposition was further explored. [Results]In 2025, the rainfall in the Jinsha River basin was lower than the multi-year average. Nevertheless, the total annual sediment deposition in the Baihetan Reservoir reached 17.77 million m³, ranking the third highest since the reservoir began impoundment and operation. In terms of spatial distribution, the mainstream deposition amounted to 12.25 million m³, accounting for 68.9% of the total, while the

tributary estuaries contributed 5.52 million m³, occupying 31.1%. Regarding storage occupation, the dead storage accumulated 12.68 million m³ of sediment, and the regulating storage received 5.09 million m³, of which the tributary estuarine deposition accounted for 20.2% of the total sediment in the regulating storage. Compared with the initial impoundment period, the frequency and magnitude of bank collapses along the mainstream have decreased notably. It is worth noting that, following the successive impoundment and operation of the upstream and midstream reservoir groups on the Jinsha River, sediment supply from ungauged areas and tributaries has become a key factor affecting sedimentation in the Baihetan Reservoir area. Meanwhile, the complex geological conditions on both banks, characterized by frequent debris flow activities, introduce considerable uncertainty and high sediment load. In particular, a single debris flow gully can deliver an annual sediment load equivalent to 8% of the total reservoir deposition, resulting in a maximum local deposition depth of 12 m downstream, thereby exerting a significant influence on local reservoir sedimentation. [Conclusions] This study identifies the spatial pattern of sediment deposition in the Baihetan Reservoir, wherein mainstream deposition dominates and tributary estuary deposition exerts a non-negligible impact. It is verified that sudden sediment yield from ungauged areas induced by debris flows serves as a critical sediment source for local reservoir deposition. The research demonstrates that reservoir sedimentation remains a prominent problem even in years with relatively low runoff, and local deposition caused by debris flows seriously endangers storage safety. The findings provide solid data support and a theoretical basis for refined sediment regulation, debris flow disaster prevention and control, and long-term storage capacity maintenance of the Baihetan Reservoir. For future reservoir operation, dynamic monitoring and comprehensive management of high-yield debris flow gullies should be strengthened to mitigate localized abnormal sedimentation and ensure the long-term efficient and stable operation of the hydropower station.