Subglacial lakes are bodies of water that form at the base of ice sheets and glaciers. Ice-surface elevation above these lakes responds to water volume changes, providing one of few ways to monitor subglacial hydrological systems. Here, we use numerical models to compare surface elevation-derived estimates of subglacial-lake length, water-volume change, and highstand or lowstand timing with their true values. Because ice flow influences the surface expression of lake-volume change, the correspondence between these estimates and their true values depends strongly on ice thickness, volume-change rate, and basal drag coefficient. For many realistic combinations of these factors, viscous relaxation of the ice-sheet surface can render lake volume-changes unobservable with altimetry. Our results highlight the need for new estimation methods that account for the effects of ice flow, as well as improvements to current resolution limitations that render some events unobservable with altimetry.