This paper examines the role of institutional arrangements as either facilitating or constraining the practice of watershed cumulative effects assessment and management (W-CEAM) within the context of the Grand River watershed (GRW), Canada. The research is based on document review, a focus group and 29 interviews conducted with academic experts, project proponents, government and watershed agencies representatives, non-governmental organization researchers, First Nations, and others with interest in the GRW. Information was gathered on existing policy and planning instruments, and relationships among the authorities and other partners that enable water resource management. Key facilitating factors for W-CEAM in the GRW include established institutions, a mature conservation authority and an ecological focus to resource management strategies, while constraining factors include obfuscation of leadership roles and lack of multi-scalar approaches to watershed science. We conclude that it is useful to conceptualize W-CEAM as characterized by both a managerial and a scientific ethos -the former facilitating the latter -and that institutional goodwill, political will and institutional capacity for innovation and creativity are additional institutional core requisites to W-CEAM.