Abstract: If delta and estuary areas are observed under the perspective of a double system of dynamic infrastructures, the object of parallel “water/urbanisation” processes, the interface spaces become key nodes. In this perspective, port and waterfront areas can be described as spaces of mediation. The article argues that in the case of Lisbon and the Tagus, as possibly in several other port cities, these edge spaces can be described as “fluid territories.” The pre-eminent characteristic of “fluid territories” is that they are not permanent, neither in space nor time. These areas present accelerated transformations, less defined boundaries, and an increased spatial and management complexity. Moreover, “fluid territories” also mediate (a) the culture-natural environment, with human action appropriating the natural system through infrastructure and urbanisation, and (b) the industrialised economic estuary, with its continuous updating. To demonstrate this hypothesis, two samples of Lisbon’s riverfront are observed, recording its constant variability over the last 200 years of industrialisation, emphasising the “fluidity” of the mediating spaces. The understanding of the “fluid” characteristic of water/land mediation spaces is relevant for the present. Being dynamic and regularly reinventing spaces, spatial planning, public space, and architectural design processes in “fluid territories” should increasingly seek adaptability, flexibility, and openness to change. In the climatic context of continuous uncertainty combined with the need to make room for infrastructure, rethinking mediation areas through the lens of the theoretical concept of the “fluid territory” enables the implementation of urban transformation processes consistent with contemporary challenges.