During the last deglaciation (21 - 7 kaBP), the gradual retreat of Northern Hemisphere ice sheet margins produced large proglacial lakes. While the climatic impacts of these lakes have been widely acknowledged, their role on ice sheet grounding line dynamics has received very little attention so far. Here, we show that proglacial lakes had dramatic implications for the North American ice sheet dynamics through a self-sustained mechanical instability which has similarities with the known marine ice sheet instability albeit providing fast retreat of large portions of the ice sheet over the continent. Systematically reproduced in the latest stage of the deglaciation, this mechanism could provide a physical origin for the debated melt water pulse 1B. Echoing our knowledge of Antarctic ice sheet dynamics, they are another manifestation of the importance of grounding line dynamics for ice sheet evolution.