The Ganges-Brahmaputra-Meghna River basin in Bangladesh faces multifarious challenges from subsidence, sediment depletion, and sea-level rise threatening its stability and resilience. Sustaining delta elevation and aggradation depends on fluvial sediment transport, yet the spatial and temporal dynamics of sediment retention remain insufficiently quantified at the system-wide scale. This study employs a process-based two-dimensional morphodynamic model using Delft3D Flexible Mesh (FM) to explore fluvial sediment transport and deposition dynamics in the Ganges-Brahmaputra River system. The model integrates hydrodynamic calibration with spatially varying Manning’s roughness coefficients derived from satellite-based land use and land cover data, and boundary conditions established using sediment rating curves. Key findings reveal that over 40% of annual suspended sediment loads are retained within the system, supporting active deltaic landforms. Sediment retention efficiency varies spatially: the Ganges retains 37.4─44.5%, primarily cohesive sediments, while the Brahmaputra retains 16.8─27.5%, dominated by fine sands. Seasonal variability dominates sediment transport, with flood season discharge accounting for approximately 95% and 80% of suspended load in the Ganges and Brahmaputra, respectively. The Padma River primarily functions as a sediment conduit, efficiently transferring suspended loads downstream with minimal retention. Bedload transport contributes 15-18% of total suspended loads, influenced by flow intensity and channel morphology, while sediment delivered to tidal deltaic plains varies considerably, driven by flood discharge duration and intensity. These insights advance our understanding of fluvial sediment budgets and geomorphic processes, equipping planners with a foundation for designing sustainable sediment management strategies, ensuing delta resilience, and enabling informed decision making for long-term river basin planning.