Climate change impacts such as shrinking glaciers and decreasing snow cover are expected to cause changes in the water balance throughout the 21st century. New proglacial lakes in recently deglaciated areas could be used for mitigation measures such as hydropower production and adaptation measures to temporarily retain water and transfer it seasonally to compensate for seasonal water scarcity. Such multi-purpose reservoirs could counterbalance the water currently provided by glaciers and the seasonal snowpack. However, new dam projects often face various conflicts due to their impact on nature, biodiversity, and the landscape. This article presents the determinants for social acceptance of the first reservoir in a recently deglaciated landscape in the Swiss Alps. Three main determinants were identified: (1) the forthcoming popular vote on the national Swiss Energy Strategy 2050; (2) the participatory process, which contains a polycentric design; and (3) the project area, which does not yet have protected status. The three determinants facilitate social acceptance of the dam project, but lead to less attention on using the stored water for multiple services. These findings have implications on sustainable development, because dams in recently deglaciated areas support the transition to renewable energy sources, but transform a natural resource system into a hydroelectric landscape.