AbstractRivers are hydrological and social entities, which complicates their use as borders but offers new conceptual possibilities for understanding border spaces. Rivers as borders challenge common understandings of seemingly static political borderlines. Moving away from simple cartographic abstractions, river borders are more than lines for territorial separation. Rivers have a material agency; they are constantly in motion and shift shape according to seasonal changes and hydromorphology. In this paper, I show how rivers through their specific characteristics, namely their materiality, movement and directionality, can challenge but also enable attempts to fix territories. Drawing on doctoral fieldwork between 2019 and 2022 along the Moselle River that crosses and builds the borders between France, Luxembourg and Germany, I argue for a dynamic and ample perspective on river borders that conceptualises them as simultaneously bounded and in motion. Using examples of contingent forms of riverine bordering this paper adds a riverine perspective to current understandings of border geographies. Seeing like a river border makes it possible to examine the relationships between territory, society and environment, and to consider material flows as fundamental to the constitution of rivers and borders.