Around 15,000 years ago, an enormous glacial dam held back a body of water known as Lake Missoula, which contained roughly half the volume of present-day Lake Michigan. As the ice age came to an end, the ice dam broke, releasing water that carved a 660ft deep, 60-mile-long canyon into solid basalt, one of the hardest forms of rock. It is also the site of a former waterfall, a horseshoe-shaped cliff that’s twice as high and three times as wide as Niagara Falls, making it the largest confirmed waterfall in the planet’s history.
http://www.bbc.com/travel/story/20170601-the-largest-waterfall-that-ever-existed
