The Mekong is the archetypal dissevered river basin, separated, divided into parts and broken up by ambiguous and undefined terms in the 1995 Agreement on the Cooperation for the Sustainable Development of the Mekong River Basin. The treaty's text generalizes and redefines the concept of a river basin in which tributaries are marginalized and headwaters are excluded, yet on paper the 1995 Mekong Agreement creates the legal fiction of a holistic water resources management paradigm primarily by focusing on the ‘imported’ (Affeltranger, 2005, p. 54) concept of sustainable development. In effect, transboundary tributaries in the Mekong legal regime are managed by each state unilaterally through application of the limiting language embodied in Article 5(A) which emphasizes hydro-sovereignty. The modern trend and badge of responsibility are to provide a legal framework for the entire watercourse, defined as including its tributaries, and to promote basin-wide governance. Analysis of selected legal regimes and case law demonstrates that ironically, despite their volumetric contributions to flow, tributaries remain at the margin of many legal regimes of international and interstate watercourses including the Mekong. The Lower Mekong River Basin states should consider use of joint development agreements (JDAs), sub-compacts or subsidiary agreements for negotiating and dealing with intrabasin water use and interbasin water diversions on tributaries.