The role played by the Mekong River in the organization of land and people is inextricably linked with a particular spatial category. The concept of the hydrological catchment extends the space of the river far beyond the limits of the river’s perennial waterbodies, to encompass vast areas inhabited by millions of people speaking different languages. Fundamental to the estimation of precipitation and water volume, areal denotations of the Mekong’s basin, delta and floodplain have been repeatedly drawn on maps by geographers, planners, engineers and cartographers. Mapped representations of the Mekong River however are not only the result of recording the flows of water, nor the domain of a single discourse. With diverging intentions, distinct and sometimes conflicting projections of the basin, delta and floodplain have prescribed the differentiation and unification of parts of mainland Southeast Asia, to articulate liquid territories that are outside a single state’s jurisdiction. As a result, the mapped articulation of surface water is reflected in the configuration of national boundaries and the arrangement of settlements. To understand how the Mekong’s catchments emerge as the geographic reference for human activities, the dissertation examines the technical and cultural notions that underpin the preparation of these maps. Drawing on the discourses of hydrology, geography, cartography as well as infrastructure design, military science, colonial politics and regional planning the research asks what territories are produced and maintained by evoking the geography of the river’s flows.