Answer: Missoula Floods
Recorded history is littered with examples of large scale flooding, but nearly all the examples you’ll find are caused by ocean-driven waters, rainfall resulting from hurricanes that make landfall, and tsunamis. Massive fresh water floods are uncommon, and even the largest recorded fresh water floods in history—such as the most destructive river flood in U.S. history, the 1927 flooding of the Mississippi River—can’t hold a candle to prehistoric floods.
That’s because today we simply don’t have the volume of fresh water needed to recreate the flooding of millenniums past thanks to the retreat of ice age glaciers. According to geological data, the largest fresh water flood occurred where modern day Oregon and Washington state sit. The floods (there were a series of them, with the largest setting this record) are referred to as the “Missoula Floods”. The floods were caused by the formation and failure of natural ice dams. Once the ice dams became unstable and broke up, the massive amounts of fresh water behind them would pour out within a few days and severely flood the surrounding region.
To put that into perspective, the failure point of the glacial lake, a region now known as “Dry Falls” in central Washington state—seen in the photo here—was five times the width of Niagara Falls with ten times the combined flow of all the current rivers in the world. The maximum flow speed approached 80 miles per hour (130 kilometers per hour) and resulted in extensive erosion to the landscape including scouring massive channels into the land and sweeping giant boulders away, piling them hundreds of feet high at the edges of the flood zone. Such levels of catastrophic flooding have not been seen again outside of the last ice age.
Image courtesy of Steven Pavlov.