Most studies in collaborative governance examine system-level or agency-level drivers in voluntary bargaining situations, but few have studied how agency-level incentives shape risk management under collaboration mandates. We argue that when mandated to collaborate, local agencies’ priority is to reduce risk by protecting their organizational autonomy and turf. These concerns can overwhelm other collaboration drivers when agencies commit to a regionally integrated framework. We test this argument by examining the implementation of the Sustainable Groundwater Management Act (SGMA), a large-scale governance reform experiment passed by the California legislature in 2014, with data derived from archival records and a statewide survey of local groundwater managers. We show that agencies are more likely to commit to a regionally integrated collaboration arrangement if (1) their mission addresses a broader issue focus (lower issue specificity), (2) they have core stakeholder groups with less concentrated interests, and (3) they have a less rigid and risk-averse culture. By theoretically and empirically unpacking the agency-specific sources of turf and reputation protection in the context of inter-agency collaboration mandates, this study contributes to both bureaucratic turf and collaborative governance research. The study also connects these two bodies of scholarship to the broader literature on collective action and common-pool resource management, providing mandate design implications for public managers.