Answer:
The destruction of Glacial Lake Missoula.
The glacial Lake Missoula was about 600 m deep and was formed in the deep valleys of western Montana, US. It existed somewhere between 13.000 to 15.000 years ago, during the Ice Age, and contained more than 2100 cubic meters of water.
When the Ice age started to lose strength, the glacial ice started to melt, which caused a flood of cataclysmic proportions, as Lake Missoula was emptied in a matter of days. The water moved to speeds of up to 100 km/h, and its portrayed in the movie as it happened at that time.