Notions of connectivity and relationality increasingly pervade theories, discourses and practices of environmental governance. Recently, the concept of the 'resource nexus' has emerged as an important new framework that emphasises the interconnections, tensions and synergies between sectors that have traditionally been managed separately. Part of a broader trend towards integrated environmental governance, nexus thinking rests on the premise that the challenges facing water, energy, food and other resources are inexorably connected and contingent. Although presenting itself as a radically new framework, the nexus discourse in current form is techno-managerial in character, profoundly de-politicising, and reinforces neoliberal approaches to environmental governance. At the same time, the 'material turn' in social science research has re-engaged ideas of social, political and material relationality to understand the complexity and heterogeneity of the socio-natural condition in the twenty-first century. Although theoretically and ontologically diverse, the fields of political ecology, assemblage thinking and infrastructure studies all critically interrogate the politics of relationality.Mobilising an urban political ecology framework, and drawing on notions of emergence and distributed agency from assemblage thinking, this research examines the politics of the water-energy nexus through a critical analysis of the extraordinary emergence of seawater desalination as a significant new urban water supply for Southern California, USA, and Baja California, Mexico. Research was conducted in the San Diego-Tijuana metropolitan region, where a large desalting facility has recently been completed to supply San Diego with purified ocean water, and a larger 'binational' facility is planned in Mexico to supply both sides of the border. The research makes three broad contributions. First, to understand desalination as emerging from the historical coproduction and urbanisation of water and energy in the American West. Second, to examine the transitioning environmental politics concomitant with calls for greater understanding of interrelationality. And third, to interrogate the efficacy of technology in reconfiguring the co-constitution of water, energy and society.