Fifteen thousand years ago, during the last Ice Age, a glacial dam collapsed in what is now northern Idaho, releasing the waters of a giant inland sea known as Lake Missoula. Five hundred cubic miles of water rampaged westward at 60 miles an hour in a torrent flowing with ten times the volume of all the rivers on earth. The flood carved canyons, gouged out enormous plunge pools, made rivers like the Snake and the Willamette run backward and scoured the earth of eastern Washington right down to bare basalt rock. The flood may have happened not just once, but many times, as the glacier periodically crept forward again to recreate the lake.
Today the landscape of the Pacific Northwest still bears the signs of these cataclysms: the flood-scoured scablands of eastern Washington, giant rocks near Portland transported all the way from Idaho by the flood, potholes and plunge pools dug by waterfalls that would have dwarfed Niagara many times over. Today both government and private organizations are exploring the idea of creating a kind of tourist trail, so that visitors to the region can learn the story of the flood, and see the enormous footprints that it left behind.