[Objectives] This study aims to quantify the respective contributions of suspended load and bed load to reservoir sedimentation over different operational periods, and to evaluate the impacts of upstream bed load supply rate, grain size distribution, and intra-annual distribution on reservoir sedimentation. The findings are intended to provide scientific support for the design, long-term operation, and numerical modeling of reservoir projects, especially in data-scarce mountainous regions. [Methods] The study takes the Tianzishan Reservoir in Hunan Province, China, as a case study. A one-dimensional water-sediment mathematical model, “HELIU-2” developed by the Changjiang River Scientific Research Institute (CRSRI), is employed to simulate reservoir sedimentation over a 300-year operational period. The model is set up using measured cross-sectional profiles along the entire reservoir reach (12.23 km, 31 sections), sediment gradation data from the dam site, and hydrological data at the dam site, with key empirical parameters adopted from established practices. Scenarios are designed to assess the influences of upstream bed load supply, bed load gradation, and intra-annual distribution of bed load supply. [Results] During the early operational period (first 10 years), bed load primarily deposits near the reservoir inlet (10 229-12 230 m from the dam), while suspended load dominates overall reservoir sedimentation, accounting for 89.78%-91.04% of total deposition over different operational years. In the near-dam reach (0-3 534 m from the dam), the proportion of bed load deposition increases gradually with operation time, reaching 40.98% at 300 years. In the inlet reach, bed load deposition reaches 100% after 100 years. In contrast, in the middle reaches (e.g., 5 884-7 110 m from the dam), the bed load deposition proportion declines after an initial increase, eventually disappearing as bed load migrates further downstream. Among the three factors examined—upstream bed load supply rate, bed load gradation, and intra-annual distribution of bed load supply—the supply rate has the greatest impact on reservoir sedimentation. A higher bed load supply rate leads to higher along-channel bed elevation and a steeper riverbed slope near the dam. The effect is more pronounced in reaches with initially milder bed slopes, where sediment deposition is more sensitive to changes in bed load supply. The influence of bed load supply rate becomes increasingly significant with longer operation time.In contrast, bed load gradation has a limited effect: coarser bed load gradation results in slightly higher bed elevation in the near-dam reach, but the overall impact on the longitudinal sedimentation profile is small. Similarly, the intra-annual distribution of bed load supply (flood-season-only versus year-round) shows negligible influence on both the sedimentation profile and bed slope. [Conclusions] (1) In the early stage of reservoir operation, bed load mainly deposits near the reservoir inlet, while suspended load dominates sedimentation in the reservoir area. As operation time increases, the proportion of bed load deposition in the near-dam reach gradually rises and reaches 100% in the inlet reach after 100 years. In the middle reaches, the proportion of bed load deposition first increases and then decreases.(2) The upstream bed load supply rate has a greater impact on reservoir sedimentation than bed load gradation and intra-annual distribution. A higher bed load supply rate leads to higher along-channel bed elevation and a steeper riverbed slope near the dam, with the effect being more pronounced in reaches of milder initial bed slope. This influence becomes increasingly significant with longer operation time. (3) Coarser bed load gradation results in slightly higher bed elevation in the near-dam reach, but its overall impact on the sedimentation profile is limited. The intra-annual distribution of bed load supply also shows negligible influence. For reservoirs with moderate sedimentation, neither factor is a controlling element.