Several studies attempt to explain how collaborative environmental governance processes operate, but the question of why collaboration relationships form has received much less attention. Motivated by this need, this paper provides insights to the broad question: why does collaborative river basin management in France depend so heavily on partnerships made up around few actors? Accordingly, our analytical framework develops a transaction cost explanation for the extent to which participatory procedure help stakeholders identify partners and initiate collaboration, and for the causal link between the attributes of these stakeholders and their partnerships. The p2 model is implemented to investigate partnership networks of the key actors that govern the management of the Gironde estuary, the study case. The results give evidence that environmental institutions bring together heterogeneous actors who might not be ready for collaboration, thereby actors' perceived power similarity; their geographical proximity and co-presence in formal fora limit transaction costs.