Many continental margins show submarine-channel patterns that resemble meandering rivers on land. How exactly do submarine channels evolve and disperse sediment and dissolved loads throughout the ocean? Do submarine channels migrate in a way that is similar to rivers? Whereas platforms like Google Earth Engine have enabled broadly accessible, global-scale analysis of rivers in Landsat imagery, this is difficult to do underwater. We used 3D seismic-reflection data to interpret the evolution of some of the largest submarine-channel systems ever recorded offshore Brazil. The margin offshore Brazil has complex topography, with many salt domes that have steered and deflected the submarine channels since the Cretaceous. We applied our experience in mapping and interpreting the time-lapse evolution of rivers in Landsat data to analyze the evolution of the submarine channels offshore Brazil. Our results show that early during the evolution of the submarine channel it migrated like a freely meandering river. However, as the submarine channel expanded toward nearby salt domes, its migration pattern fundamentally changed. With nowhere to expand further, the submarine channel translated downstream to produce a pattern characteristic in rivers that are confined by banks resistant to erosion.