Both civil engineering and environmentalism strongly influenced the development of water governance in the Netherlands in the 20th century. Much research has focused on these aspects separately. This article maps the interaction between governance, technology and ecological systems in the Netherlands, to provide insights into how these are co-evolving. The analysis is based on a combination of a literature study and an empirical case study on the debates concerning the reopening of the Philipsdam, in the Southwest Delta of the Netherlands. It shows how the negotiations that took place in constructing facts in the Philipsdam case both increased the complexity of decision-making concerning the dam itself and radiated outwards to affect other parts of the Dutch water system. We conclude that the process of constructing facts and the way these are framed once they have been established as facts are both intrinsically political and reflect the multiplicity of views of how the lake works and what the problem is, and how these views are incompatible at times. As such, ontological complexity is ingrained in what is represented as facts and severely complicates an apparently matter of fact decision to reopen a dam.