Abstract The interface between rivers and lakes forms a unique hydraulic continuum, in which lotic and lentic regimes dynamically interact and transition between each other. A common expression of this lake‐river continuum (LARIC) is reservoir backwater, where water surface gradually rises from the reservoir pool upstream to the free‐flowing river. Observing this nuanced hydrodynamics is challenging but critical for understanding fluvial‐lacustrine connectivity. Here, we couple open‐channel hydraulic theory with water‐surface profiles derived from Surface Water and Ocean Topography (SWOT) satellite data to characterize the hydrodynamic zone along river‐to‐reservoir transitions. Our prototype framework adaptively delineates LARIC transition zones, classifies regime types (M1 backwater and M2 drawdown), and quantifies their longitudinal and vertical extents. The characterized hydrodynamics agree with theoretical expectations and reveal pronounced seasonal variability in response to reservoir regulations. Together, these results demonstrate SWOT’s unique potential for global LARIC characterization and underscore its value for lake‐river science and water management.