The largest floods known to have occurred in human history are those from the end of the last ice age, between 13,000 and 8,000 years ago. As the world's great glaciers began to melt, they left large freshwater lakes behind that had been precariously confined by retreating ice dams. When those ice dams failed, the floods that resulted were almost beyond comprehension.
The largest of these floods are estimated to have had peak discharges of nearly 20 million cubic meters per second. The Altai Flood, largest of them all, swept through what is today central Russia. Giant current ripples, like the patterns found in river sand but large enough to form hillsides, still undulate over the Kuray Basin (pictured here), evidence of an unfathomable flood.
North America was also home to floods of comparable size. A series of cataclysmic floods, called the Missoula Floods, regularly swept through what is today the Columbia River Gorge. Meanwhile, floods that resulted from the draining of the ancient Lake Agassiz — a lake so massive that it held more water than contained by all lakes in the world today, combined — are estimated to have risen sea levels worldwide by as much as 9 feet.