How is water engaged in, or mobilized for, the production of social difference and indifference? This article proposes the rubric of ‘urban waterlines’ to examine the political agency of various urban waters in inscribing categories of personhood and effects of discrimination. Waterlines are conceived as material (socio-technical and historical) formations that set out to channel, contain or accelerate water, but are ultimately effected by pushbacks, overflows and outbreaks, or the subsidence and vanishing of water. They are dialectically assembled by the workings of water in two registers: first, as a variable form shaped by historically shifting valorizations of water as resource, factor of production, object of consumption, aesthetic feature, or waste; and second, as a relatively autonomous natural element that resists or exceeds these shapings. The article discusses three kinds of urban waterlines—boundaries, flows and infrastructures—using case studies from Chennai to illustrate how each is assembled through socio-natural, technological and discursive operations and works to dispossess or disadvantage specific people and places. It brings Urban Political Ecology’s analyses of water forms as historically crafted socio-natural assemblages into conversation with anthropological scholarship on how indifference is produced through the everyday workings of the state.