<p>A comprehensive assessment of hydropower’s climate impact requires considering both greenhouse gas (GHG) emissions and carbon burial in sediment. This study examines the Wujiang River in southwestern China, where seven cascade reservoirs were categorized into upper (Group 1), middle (Group 2), and lower (Group 3) reaches according to their geographic locations and proximity to key sediment source areas. The G-res Tool was conducted to simulate greenhouse gas (GHG) emissions from these reservoirs, which ranged from 2,231 to 19,774 t CO_2e yr⁻¹ after impoundment. The primary influencing factors include reservoir age, surface area, and water retention time. Due to the steep mountainous terrain, deep valleys, and pre-impoundment clearing, the GHG emissions from these groups of reservoirs are lower than those of other reservoirs at similar latitudes worldwide. As the cascade reservoirs were gradually constructed, sediment accumulated behind the dams, leading to the long-term storage of terrigenous carbon. Notably, the upstream Group 1 reservoirs exhibited an exceptionally high terrigenous carbon storage rate during their early impoundment phase (7,468 gC m⁻² yr⁻¹), largely due to significant sediment input from the upstream Yachihe Basin, a major sediment source. In contrast, the downstream reservoir (Group 3) exhibited relatively smaller total terrigenous carbon storage rates (319 gC m⁻² yr⁻¹), corresponding to reduced sediment retention. Despite variations in sediment retention among the groups, the total terrigenous carbon storage of all groups of reservoirs (ranging from 43,020 to 999,403 tC yr⁻¹) exceeded their post-impoundment GHG emissions. By integrating carbon emissions and sedimentary carbon sequestration across a cascade system, this study provides a system-scale carbon balance assessment of hydropower reservoirs. Our results indicate that the cascade reservoir systems, like those on the Wujiang River, function as a net terrigenous carbon storage, mainly due to the large sediment sequestration. These findings highlight the importance of incorporating both carbon emission and storage into carbon accounting frameworks and provide new insights for carbon balance assessment and management of hydropower systems.</p>