Reservoirs are significant sources of greenhouse gases (GHG), but their contribution to global C emissions remains uncertain. Part of this uncertainty is due to limited empirical data on the mechanisms underlying reservoir C fluxes and their variability at a local scale. This lack of information is especially apparent in tropical regions, where reservoir C emissions tend to be the highest. This project focuses on understanding CO2 and CH4 dynamics in Batang Ai (Malaysia), a tropical reservoir. The first objective is to assess the different C emission pathways (diffusion, ebullition, and downstream emissions) and how they vary in space and time to shape the C footprint of the reservoir. The second objective is to study how metabolic C processes, underlying CO2 and CH4 fluxes, vary in the water column and sediments along the river to reservoir gradient. The final objective is to quantify the vertical transport of C gases in order to understand the role of physical processes in sustaining surface fluxes.