Collaborative environmental governance is increasingly being used by public administrators to integrate divergent sectoral interests and deliver public goods that individual organizations would fail to deliver on their own. Yet, empirical studies on how exactly collaborative governance leads to integrative outputs remain scarce. This study applies a process-tracing methodology to test the hypothesized causal mechanism of collaboration dynamics leading to integrative output in a case of collaborative flood risk management from the Netherlands, the case of Grebbedijk. By drawing on multiple data sources, the analysis validates the mechanism and confirms that the dynamic interaction of highly functional principled engagement, sufficient shared motivation and a wide range of capacities for the joint action is a causal process linked to the successful output in the studied case. However, it also demonstrates that a set of pre-determined capacities for joint action, particularly initiating leadership, procedural arrangements and resources, were critical for the mechanism to unfold successfully. The findings of this study also suggest that well-organized processes of principled engagement facilitated by adaptive and connective leaders may compensate for lack of shared motivation among collaborating parties and succeed in delivering desired collaborative outputs without investing much in building trust and shared motivation.