River damming can significantly alter the hydrology and nutrient levels of river water, resulting in substantial greenhouse gas (GHG) emissions to the atmosphere. However, the dynamics of greenhouse gases in the discharged water downstream of dams remain poorly understood, despite being recognized as a crucial source of GHG emissions in river‐reservoir systems. In this study, we conducted comprehensive measurements of GHG concentrations and water chemistry in a large subtropical reservoir and its upstream and downstream rivers to investigate the spatiotemporal patterns of GHG concentrations and fluxes and to identify their governing mechanisms, with a primary focus on downstream GHG dynamics. Our analysis revealed that the distribution of pCO2 among the reservoir and its upstream and downstream rivers was predominantly controlled by aquatic metabolism and atmospheric CO2 exchange. Conversely, the distribution of CH4 and N2O levels was largely influenced by anaerobic metabolism. Seasonal fluctuations in GHG dynamics were linked to hydroclimatic conditions, including water temperature, hydrologic connectivity between land and rivers, and reservoir thermal stratification. Anthropogenic activities (e.g., agricultural land use) were found to affect the downstream trend of GHG concentrations. Higher GHG fluxes in the downstream rivers compared to reservoir were attributed to the anaerobic production of CH4 in the reservoir and increased gas transfer velocity in the downstream rivers. These findings underscore the critical influence of anthropogenic activities on downstream GHG dynamics and emphasize the necessity of integrating anthropogenic impacts and seasonal variability in downstream GHG emissions to enhance our understanding of the carbon budget in river‐reservoir systems.
