Rethinking the Filling Process of Mega‐Dam Cascades

Abstract The filling process of large hydropower reservoirs is an often‐overlooked process that can attract most of the attention and criticisms of a dam's lifetime. Understanding what drove past filling efforts and exploring trade‐off solutions is therefore critical for regions where hydropower development is flourishing. Yet, there is a lack of methodological frameworks applicable across different contexts (e.g., existing and planned systems) that can characterize how trade‐offs evolve under varying hydro‐climatological conditions. Here, we focus on the filling strategies of the Upper Mekong hydropower system, one of the largest mega‐dam cascades (capacity 1,000 million cubic meters) built in the century. Our ex‐post analysis for the period 2008–2016 reveals that the historical filling strategies have largely prioritized electricity production at the expense of downstream hydrological alterations—peak discharge decreased by more than 20% during the filling of the largest dams. On the flipside, there are tangible opportunities for designing better trade‐off solutions that would be appealing to all riparian countries and that are robust to different hydro‐climatological conditions. Key to this step is the coordination of the filling process of the largest storage facilities.