The largest flood ever is thought to have occurred approximately 50,000 years ago in an area that is now Idaho. The flood is throught to have resulted in the creation of geological formations known as Woody's Cove and Stubby Canyon.
Glacial lakes are formed when a glacier serves to dam melt water. Lakes that suffer the collapse of an ice wall create what is called an outburst flood. According to Caltech, the largest flood to ever occur is thought to have taken place about 46,000 years ago. The rate of water flow from this event would have been several times greater than any flood since historical records began.