Conflicts over water resources are common on international rivers. To study the impact of upstream dams on downstream countries when coordination is limited, we examine a unique setting where China built massive dams along the Mekong River sitting right at its border with downstream countries. Using Machine Learning and Difference-in-Difference identification strategies, we find that, on average, upstream dams smooth downstream water discharge and levels across seasons, and improve downstream agricultural efficiency. Additionally, we conducted two event studies to better understand the impacts of dams facing water scarcity. The results suggest that the lack of alignment between upstream and downstream countries regarding the objectives of dam operation can enlarge damages during extreme weather events. The finding of this paper indicates the importance of upstream countries taking responsibility to ensure effective transboundary water management, particularly in the face of climate change.