Younger Dryas

The only hypothesis that stands, to explain the rapid glacial interlude, is that of the Mississippi and St. Lawerence meltwater routing.
Routing of water melting from the southern perimeter of the Laurentian ice sheet. Initially, almost all the water went down the Mississippi into the Gulf of Mexico. Then in a series of steps it was diverted to eastern outlets. The last of these late-glacial diversions occurred when the Lake Agassiz drainage was shunted across Canada to the Kirkfield outlet into the St. Laurence estuary, at this point all drainage was to the east. Following this, a readvance of the ice in the Lake Superior sector blocked the eastward flow of Agassiz water forcing it once again to flow to the sea via the Mississippi River.