In recent years, threatened deltas have emerged as a significant matter of concern in numerous fields. While Earth System science and social-ecological systems focus on topics like global water circulation and sediment transport, social scientists tend to consider the problems facing particular deltas in the context of modernization or (post)-colonial development. There is nevertheless broad agreement that the delta crisis raises fundamental questions about modern approaches to infrastructure planning. Thus, environmental and sustainability scientists have come to recognize “the social” as integral to the delta crisis. This understanding of “the social,” however, takes two quite different forms. As an object of social-ecological systems research, the social is modeled alongside ecological systems. However, as a context for scientific interventions in environmental policy it appears as an obstacle to achieving sustainable delta policies. Based on a careful examination of Earth System science and associated discourses, we show that this instability of “the social”, combined with the ambition to integrate ‘it’ in an encompassing system poses serious problems for interdisciplinary delta research and for more imaginative and inclusive collaborative efforts to tackle the delta crisis—including, but going considerably beyond, policy and governance. Rather than integrative systems, we argue that the situation requires the creation of sophisticated conjunctions of epistemologies, methods, and practices. Such conjunctions, we suggest, pave the way for a cosmo-ecological approach, where social, environmental and sustainability sciences work together with designers, urban planners, policy-makers, and affected or concerned citizens on solving multi-scalar delta problems by working across their differences.