Surface rivers can transport meltwater from floating ice-shelves into the ocean, limiting ponding. However, if a river channel deepens to sea level, seawater can flow up into it and form an ice-shelf estuary. This adds load to the ice shelf and increases stresses that weaken the ice. Using high-resolution satellite imagery, we develop a new way to measure how quickly these rivers deepen, which we use to constrain the timing of Petermann Ice Shelf’s estuary formation over multiple melt seasons.