ABSTRACT The paper examines source-to-sea (S2S) systems under international law from the perspective of the community-of-interests doctrine. It positions S2S systems alongside the drainage basin approach, which delimits the Doctrine’s primary biogeographical scope. Community-of-interests is based on the physical unity of a drainage basin, which creates multidimensional interdependence between states, and thereby gives rise to common interests that tend to require catchment-wide joint cooperation. On the basis of treaty and case law, the paper investigates the extent to which the Doctrine, premised chiefly on the basin, may contribute to the sustainable management of S2S systems.