New regulatory water management requirements on an international level increasingly challenge the capacity of regional water managers to adapt. Stakeholder participation can contribute to dealing with these challenges because it facilitates the incorporation of various forms of knowledge and interests into policy-making and decision-making processes. Also, by providing space for informal multi-stakeholder platforms, management experiments can be established more easily in rigid regulatory settings, allowing for social learning to take place. Stakeholder participation is currently stipulated by several legal provisions, such as the Water Framework Directive, which plays an increasingly important role in European water management. Drawing on recent experiences in a participatory process in the German Dhuenn basin, a sub-basin of the river Rhine, we explored the interplay of informal and formal settings in a participatory process. To what degree can we allow for openness and catalyze social learning in participatory processes grounded in formal management structures? To what degree can results of informal processes have an impact on practice? We analyzed three major challenges related to this interplay: (1) the niche-finding process to establish a participatory platform; (2) the co-design process by water management practitioners, researchers and consultants; and (3) the tangible outputs and learning. We found that niches for the establishment of informal participatory platforms can occur even in a rigid and strongly structured administrative environment. Further, our case study shows that collaborative process design fosters dealing with uncertainties. We conclude that in an effective participatory process, a balance should be struck between informality and formal institutional structures to catalyze experimentation and learning and to ensure that process results have an impact on management decisions.