This article investigates the making of Hong Kong's water supply system since 1959. It starts by assessing the perspectives provided by the regime approach and the political ecology literatures. The case of Hong Kong brings in ideas from border studies and draws attention to the changing nature of the border to explain socio-ecological and scaling interactions. The case study maps the border relationship between China and Hong Kong (and Britain), and the political tussle between them over the control of water supply to the city in the late colonial period 1959–78, which resulted in the creation of a localized self-sufficient water supply system in Hong Kong, and the consolidation of Hong Kong's scale as a colonial city-state under British rule. It further explicates the change in the nature of the political border since 1979, and the processes by which Hong Kong abandoned attempts to strengthen its local supply, becoming dependent on supply from the regional Dongjiang water networks, as well as the transformation of its scale to become a subordinate of the larger political unit in subsequent years.