Our current concepts of abrupt climate change are strongly influenced by compelling palaeoclimate evidence for events like the Younger Dryas, in which massive changes in climate occurred essentially instantaneously. It is generally thought that an injection of freshwater from the retreating Laurentide Ice Sheet altered the Atlantic Meridional Overturning Circulation and triggered the Younger Dryas, but convincing geological evidence to support this theory has, to date, proven elusive. Now Julian Murton and colleagues at last identify a major flood event that is chronologically consistent with the Younger Dryas. Gravels found above an erosion surface in north-west Canada are consistent with the major flood path running through the Mackenzie River into the Arctic Ocean, rather than south along the Mississippi River or east through the Great Lakes.