The last of the North American proglacial lakes, north of the present Great Lakes, reached its largest volume around 8,500 years ago, when joined with Lake Agassiz. But its outlet was blocked by the great wall of the glaciers and it drained by tributaries into the Ottawa and St. Lawrence Rivers far to the south. About 8,300 to 7,700 years ago, the ice-dam failed catastrophically. Lake Ojibway's beach terraces show that it was above sea level. The volume of Lake Ojibway is estimated to have been about 163,000 cubic kilometres, more than enough water to cover a flattened-out Antarctica with a sheet of water deep. That volume was added to the world's oceans in a matter of months.