A Review of the Impact of River Hydraulic Engineering on the Carbon Cycle

River hydraulic engineering plays a substantial role in maintaining the water-energy-food-economy nexus and coping with climate change. The co-construction of carbon peaking, carbon neutrality goals, and ecological civilization presents new demands for the development of river hydraulic engineering. Building upon an overview of the development of river hydraulic engineering, particular attention has been paid to its impact on the carbon cycle process. A review of the research on carbon cycling indicates that the carbon cycle of river hydraulic engineering is primarily influenced by geographical location, project scale, and operational management, whereas the evaluation results of carbon balance are significantly affected by differences in calculation methods, evaluation systems, and data discrepancies. Analysis of the carbon balance of reservoirs nationwide also revealed that China's reservoir CO2 flux was slightly higher than the world average, whereas the CH4 flux was lower. The total carbon storage in reservoir sediments in China is approximately 2.103 billion tons, rising at a rate of 76 million tons per year. Estimation results of carbon emissions reduction in hydropower projects indicate that water conservancy projects have significant functional benefits and demonstrate a noticeable "carbon sink" effect on the energy structure. Assuming that hydropower is entirely replaced by thermal power, hydropower reduces approximately 3,984 Tg (CO2eq)/a of carbon emissions from the power system, of which small hydropower accounts for approximately 6%. This study also explored policy-oriented issues in the development of river hydraulic engineering, assisting in elucidating the challenges and demands faced by river hydraulic engineering during the period of multi-objective co-construction. These challenges include balancing ecological friendliness to enhance carbon sequestration, ensuring food security, harnessing water resources for economic purposes, maintaining policy continuity, and increasing government credibility. Recommendations for the future include ensuring policy continuity, deepening research on enhancing data sharing, conducting fundamental research, and strengthening multiscale integrated systematic studies. It is necessary to establish a unified carbon balance evaluation system, conduct in-depth research on process-based mechanisms and cumulative effects, and explore the synergy between research, application, and policy development. These efforts will actively promote the innovative development of river hydraulic engineering within the context of ecological civilization construction and the achievement of the "dual carbon goals".