In complex, polycentric environmental governance systems, actors may choose to collaborate with one another to reduce their collective vulnerability and enhance system function. However, collaboration can be costly, and little evidence exists for how particular collaborative forums impact the broader governance system in which they are embedded. To address this gap, we investigate the role of intermediate collaborative forums, which support collaboration among a subset of system actors, in polycentric governance systems. Empirically, we analyze the structural and functional role of an intermediate collaborative forum called the Arizona Municipal Water Users Association (AMWUA) within the municipal surface water governance network for the Phoenix Metropolitan Area (PMA) in Arizona, United States. To do this, we draw from 21 interviews with water professionals in the PMA, which we analyze through a combination of network analysis and qualitative coding. We find that AMWUA facilitates strong bonding capacities among members, allowing for streamlined bridging to the rest of the network that enhances information processing and advocacy of member needs. Our findings advance theory on the role of collaboration in polycentric systems and inform the design of collaborative institutions to improve environmental governance.