The Peterborough–Trenton drumlin field and associated meltwater erosion marks (s-forms) were interpreted as the products of erosion by turbulent separated flows within a catastrophically released subglacial meltwater sheet, the Algonquin event. Recent theoretical modelling of ice-sheet hydrology suggests progressive channelization of meltwater during the collapse of such a sheet. Inductive testing of this hypothesis using observations in south-central Ontario forms the crux of this paper. A geometric model of the interaction of rough ice base and bed surfaces gives inferred meltwater flow patterns. Such a rough interactive geometry is suggested as the bed is drumlinized, and flows responsible for drumlins may be expected to have eroded similar forms into the ice base. Three landforms in south-central Ontario; namely tunnel channels, megachannels, and late-stage sheet-flow scours, are described, and interpreted as evidence of progressive channelization and flow diversion processes, governed by the geometric interactions between the recoupling ice base and its bed and thermodynamic feedbacks within an increasingly discontinuous meltwater sheet. Some fields of transverse ridges within tunnel channels may record meltwater deposition towards the end of the Algonquin event. By contrast, eskers within tunnel channels record a return to seasonally driven meltwater drainage. The Dummer Moraine and the Oak Ridges complex are integrated into a new landform–vent sequence for south-central Ontario which provides a framework for further field testing.