The total storage capacity of the reservoirs in Switzerland represents only 42% of their annual production of electricity, but secures approximately 100% of hydropower’s contribution to the winter electricity supply. In order to avoid spilling, the storage power plants therefore need to turbine the supplementary inflows like a run-on-the-river plant when the reservoirs are full at the end of the summer semester. This leads to a significant need to import electricity during the winter semester due to lack of production. The storage capacity of the reservoirs located in Valais contains more than 30% of the total live storage capacity in Switzerland and is strategical for the seasonal energy transfer. This paper presents a methodology to assess the potential to increase seasonal storage and then explores this methodology on a case study. The methodology was first developed and tested using recent hydrology and reservoir operation, and then used to explore alternative ways of operating and adapting the hydropower infrastructure when exposed to predictions of future runoff for the coming decades. In fact, from now to the year 2100, the inflows to these reservoirs are expected to vary considerably both in time and space. For instance, an especially significant increase of the yearly inflows due to glacier recession is expected for the 2045-2074 period. This will lead to new opportunities and new challenges in the operational management of the dams. The application of the methodology to the case study of the hydropower scheme of Grande Dixence illustrates two different options to increase the seasonal storage capacity: the creation of a new reservoir further upstream and the heightening of the Grande Dixence dam. The optimal storage capacity that guarantees the best use of the available future inflows and an adequate balance between water abundance in some years and water scarcity in others is computed considering different ways of using the inflows in a range of consecutive years and runoff predictions obtained for three greenhouse gas emission scenarios.