Abstract: Ports worldwide are shifting from their original locations, and the reasons behind these patterns of port development are multifaceted. Reasons for locational changes may include local factors such as natural conditions, or global trends like containerisation. This article argues that flows play a significant role in making and breaking metabolic relations between spaces. The authors use a combination of qualitative and quantitative approaches to characterise the evolution of port and territory interactions. A historical sequencing illustrates the successive phases of connection and disconnection between port and non-port spaces over the years. Drawing from the urban metabolism framework, the analysis of a port’s traffic structure demonstrates how flows influence a port’s extraterritoriality. For this research, the case of the Loire estuary was chosen: the Grand Maritime Port of Nantes Saint-Nazaire is a polycentric port that originated in Nantes and extended coastward in Saint-Nazaire. The case study reveals that a port reaching an urban area does not necessarily mean it will engage or support metropolitan development. Moreover, it concludes that flows are active drivers of territorial development in port regions. The research more broadly discusses the extraterritoriality of large logistics and transport infrastructure, like that of ports.