Following the 1953 flood, the Dutch delta has been shut off from the sea by a series of dams and dykes. This closing-off transformed the delta from a constantly changing estuarine environment into a freshwater body. A different way of thinking about ‘nature’ in the period 1970–1980 led to the policy proposal to transform the Haringvliet, one of the shut-off water bodies, back to its natural state. The intended first step was a minor change in the sluice control of the dam that separates the Haringvliet from the sea. However minor, this step was never taken and it is still an ongoing policy debate whether the sluice should be slightly opened or not. This paper presents a coevolutionary analysis of why such a seemingly small change could take so much time and effort to become actualized. We demonstrate that the course of the policy process is determined through patterns of reciprocity between the physical and societal systems. Effects of measures are unpredictable, and the desired state of this water basin greatly varies between actors and over time. It is a combination of these factors that explains why the sluice control has not been changed yet.