During the last ice age, 20,000 years ago, Canada and North America were covered in sheets of ice 3-4 km thick by the Tioga glaciation.

When the glaciers began to melt, the meltwater flowed down the river courses which had been buried under the ice. In Montana, the Clarke Fork of the Columbia River drains 500 km of central Montana and the Rocky Mountains. The course of the river was blocked by a huge plug of glacial ice 610 m high, forming an impervious dam. Behind the dam, a large portion of Montana filled up with water, forming the gigantic Lake Missoula. 320 km long, and with a surface area of 7,770 km², it contained 2,500 cubic kilometres of water (about half the volume of Lake Michigan).

As the water rose, the pressure of the water pushing against the bottom of the ice dam became enormous. The pressure changed the melting point of the ice, and cracks started to form. The ice dam eventually suffered a catastrophic failure and the entire mass of Lake Missoula behind it was suddenly free.

Imagine a wall of water 120 m high, travelling at 105 km per hour. This monster roared along the course of the Columbia River until it was forced into a narrow gap in the gorge. Here it gouged away the hole to make a path for itself. The gap we can see today is 5.5 km wide. Pouring out of the hole, the water fell down a 130 m drop, forming history’s largest known waterfall, with ten times the output of all Earth’s current rivers combined.

Dry Falls, the 5.5 km wide gap torn out by history’s largest waterfall.

The kinetic energy of the massive flow of water racing through the waterfall was more than 2 terawatts (TW) per hour – enough power to provide electricity to the entire globe.

Large parts of Idaho, Washington and Oregon were flooded 50-60 metres deep in just a few days.

When the lake had emptied, there was nothing to stop the ice dam pushing into the Columbia River again, so the lake could reform, and the catastrophic cycle repeated itself approximately 40 times over the next 2000 years.

North America at the time was inhabited by Native American people who may have witnessed the floods first-hand.

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