Conflict over new dams and reservoirs is well-studied, but less is known about controversies over the reoperation of existing water infrastructure. This paper presents two cases of reoperation that have been embroiled in conflict: the Gross Reservoir Expansion Project in Colorado, United States, and the Yesa Reservoir Regrowth project in Aragon and Navarra, Spain. A historical analysis of each of the cases relies on process tracing, reaching back to their construction in the 1950s up to present day, and a cross-case comparison distills findings on the causes of historical and contemporary conflict. The paper adds empirical evidence to the phenomenon of path dependency, and argues that a transition of the dominant mechanism of institutional reproduction occurs in the cases—from power to legitimacy—which in turn informs us about the historical and present-day conflicts. We find that through the contemporary reoperation, water service providers are experiencing a legitimacy crisis related to the quickly-evolving values of water users, and their access to competing sources of knowledge and expertise.