[Objective] This study aims to address the insufficient understanding of long-term, cumulative sedimentation patterns in downstream water-source reservoirs under the influence of basin-wide cascade hydropower development. Specifically, it seeks to quantify how upstream cascade interception alters the sediment influx, gradation, deposition morphology, and capacity loss of the Benzilan Reservoir, which serves as the water source for the Central Yunnan Water Diversion Project. The research also provides a comparative evaluation of low-dam and high-dam schemes to inform optimal project selection and long-term operational safety. [Methods] A one-dimensional mathematical model for total sediment transport (HELIU-2) was developed based on non-equilibrium transport theory. The model incorporates simplified formulations of the flow continuity, momentum, and sediment continuity equations, and separately accounts for suspended load and bedload transport. The computational domain covers a 78.71 km reservoir reach, discretized into 33 cross-sections. Due to the paucity of historical bathymetric data in the study area, model verification was conducted against the quasi-equilibrium long-term evolutionary trend of the Jinsha River channel, inferred from cross-sectional surveys and local expert knowledge, rather than against a conventional measured erosion-deposition dataset. Four scenarios were simulated over a 100-year operational horizon: a low-dam scheme (normal water level 2 090 m) and a high-dam scheme (2 150 m), each considered both with and without the trapping effect of upstream cascade reservoirs. The sediment interception efficiency and the resulting downstream gradation modification were estimated using the equilibrium slope method, with cascade commissioning scheduled in a phased sequence over time. [Results] Upstream cascade interception exerts a dominant effect on the sedimentation regime of the Benzilan Reservoir. Compared with scenarios without upstream trapping, the 100-year total deposition volume is reduced to merely 2.85% for the low-dam scheme and 4.27% for the high-dam scheme. Concurrently, the intercepted sediment becomes substantially finer, as coarser fractions are preferentially retained in upstream reservoirs. This combined reduction in sediment load and grain size fundamentally alters the deposition pattern: the typical delta progradation observed under natural inflow conditions is replaced by a thin, band-shaped layer of fine sediment accumulating near the dam. For the low-dam scheme with upstream interceptio, the 100-year deposition volume is 0.054 billion m³, with a trap efficiency of 98.97%, a capacity loss of only 1.55% (all from dead storage), and a near-dam bed elevation of 2 014.86 m. For the high-dam scheme with interception, the corresponding values are 0.291 billion m³, 93.87%, 1.88%, and 2 017.39 m. In contrast, without upstream interception, the low-dam scheme exhibits delta-front advancement to the dam by the 50th year with a 54.54% capacity loss, while the high-dam scheme maintains the delta apex 17 km from the dam by the 100th year with a 44.11% capacity loss. [Conclusion] Upstream cascade interception drastically reduces sediment inflow to the Benzilan Reservoir. Incoming sediment also becomes substantially finer, as coarser fractions are preferentially retained upstream. This combined reduction in load and grain size transforms the deposition pattern from a prograding delta to a thin, band-shaped layer near the dam, allowing most fine sediment to be transported through the reservoir and flushed downstream. Consequently, near-dam aggradation and operational capacity loss are markedly reduced under both schemes. Comparatively, the high-dam scheme exhibits superior long-term capacity retention, lower near-dam bed-elevation rise, and better sediment regulation under identical upstream conditions. Quantitatively, this work fills a critical gap by systematically quantifying the cumulative interception effect of a multi-reservoir cascade on a downstream water-diversion source, demonstrating that traditional predictions based solely on natural inflow conditions would severely overestimate actual sedimentation risks. However, extreme flood events and consolidation of fine cohesive deposits may introduce additional uncertainties, warranting further investigation.