Megaflood

In the vast, arid landscape of Eastern Washington lie the traces of an ancient disaster. Outside the city of Spokane, massive scour marks run through the rocky ground, creating a strange terrain known as the scablands. A bit to the west, a channel has been shaped into the Earth that's as deep as a forty story building. Elsewhere, miles of rolling hills run across Washington, Montana, and Idaho, resembling enormous ripples up to 15 meters high. These characteristics are all the lingering remains of an epic geological mystery that took nearly half a century to solve. Every great mystery requires a great detective and geologist J Harlen Bretz was a great detective indeed. In the early 1900s, He investigated these strange features and soon concluded that features like these could only have made by water. A lot of it. Running fast. But that stream of water that had transformed the land would have to be unimaginably huge. It must've been a flood, of almost biblical proportions. He met scepticism, to put it softly when Bretz presented this hypothesis in 1927. But ultimately, his investigation would unravel one of the most powerful and bizarre mysteries in recent geologic history. And as a result, it would change the way geologists understand the world today. Because Bretz was right: This landscape was the result of flooding. But not just a single flood. Rather, it was dozens of major, destructive floods that took place over the course of more than 7,000 years, forever transforming the landscape of the Pacific Northwest. What Bretz had discovered was evidence of floods that can only be described in one word: catastrophic.

When Bretz first started studying the weird landscape of the Northwest in the 1920s, there was a certain school of thought that most geologists followed. It was known as uniformitarianism, the idea that the present is the key to knowing the past. In this view, all rocks, landforms, and other geological features can only have been created by processes that we can observe today. And except for the occasional volcanic eruption, or river overflowing its banks, all modern processes are gradual, like erosion. So to these geologists, the scablands of Washington could only be formed by glaciers and the ripples must be deposited of what the glaciers had slowly scraped away. Because of the results of glaciers had been seen around to world and through the lens of uniformitarianism, they seemed to most closely resemble the features that Bretz was studying. But Bretz had studied glacial geology, too, and he knew what glacial could do. And to him, the characteristics he saw just didn't fit. Rather, they looked like a scaled-up version of what appears after a big flood. For Bretz, the clearest evidence of flooding was the shape of the canyons in the Scablands and other areas. These canyons, also called coulees, have flat bottoms and steep, vertical walls - very different from the U shape of valleys that are carved by glacial, or the V-shaped valley made by rivers. One especially large coulee called Dry Falls appeared to have formed a massive waterfall over 100 meters tall and 3 and a half kilometres wide; that's twice as tall, and five times wider, than Niagara falls! But water doesn't just remove things; it also deposits things. And Bretz saw that the landscape was scattered with boulders weighing up to 200 tons, having tumbled miles away from their origin, like pebbles on a beach. He also noted massive ripples in the earth and gravel bars up to 90 meters high, all types of deposits made by powerful flowing water. Finally, Bretz knew that these features couldn't be linked to glaciers, because of what was missing: the huge ridges of deposited sand and gravel called moraines, which form around advancing glaciers. Only one tiny moraine was located in the scablands, not nearly enough evidence for the giant glaciers that would have been required to carve features this big. But despite all of this evidence, other scientists weren't convinced that this strange landscape was developed by an epic flood. They argued that humans had never observed a flood anywhere that big as the one that Bretz proposed, so they were unwilling to believe that such a thing was even possible. Uniformitarianism explained a great deal about geology and epic floods just didn't fit into it. What giant floods did fit into was the geological mindset that Uniformitarianism had replaced: An older school of thought known as catastrophism. Catastrophism was an idea put forward in the early 1800s by French scientist Georges Cuvier. This theory explained all geologic formations as evidence of large, sudden, and unpredictable events usually that was referred to in the bible like celestial impacts, enormous volcanic eruptions, and massive floods. So no matter how good his evidence was, Bretz's hypothesis seemed extremely outdated. And there was still one mystery that Bretz couldn't explain.  If all this flooding really happened, then where's the water come from?

He originally thought that the water had come from some melting glacier. But he couldn't explain how the glacier had melted fast enough to create so much water all at once. It turns out, he was looking in the wrong area. But someone else knew where the water came from. This half of the mystery was solved by Joseph T. Pardee, a geologist with the U.S Geological Survey. Pardee had visited a conference where Bretz presented his hypothesis about the Ice Age megaflood and watched as Bretz supported his claim against a room full of sceptics. And more than 10 years earlier, Pardee had been working in Western Montana and where he'd found evidence of an enormous, Ice Age lake that had since disappeared. His main piece of evidence? Distinctive lines he saw high on the hillsides. These lines create small benches, much like the shorelines of a reservoir. So Pardee thought that these ancient shorelines were made by an ancient lake whose origin was the Clark Fork River, which still flows today through the valley below. This giant lake came to be known as Glacial Lake Missoula, named after the town. But a reservoir requires a dam, and a lake this size would've needed a big one. So what had dammed the river to form a lake, what happened to the dam?

To find out, Pardee followed Lake Missoula shorelines for miles to the west, into the panhandle of Idaho, at which point the lines just disappeared. But where they ended, he found something else: big, U shaped valleys and glacial moraines both evidence of glaciers in the area. So the evidence proposed that a glacier had blocked the river to form the lake. Judging by the landforms around it, it must've been about 50 kilometres wide and more 600 metres tall. And the reason it didn't exist anymore was that it was made of ice. So with his missing dam now found, Pardee had a new question to answer: where'd all of the water go? By some accounts, Pardee had already suspected that the scablands that Bretz described were created by the drainage of his lake. But it took more than a decade for Pardee to publish the evidence that linked his lake to Bretz's flood. On a mountain pass in northern Washington for example, he found massive scour marks. In the river valleys of western Montana, he recorded large bars of debris that had been carried there by currents. And in Montana and Idaho, he studied enormous rippling dunes made of gravel. All of these strange features were consistent with the evidence of flooding. And they were all downstream of where the ice dam would have been. So Pardee concluded that, periodically, too much water built up behind the ice dam that held back Glacial Lake Missoula, until it ruptured. After all, ice is less dense than water. So when the water level in Lake Missoula got high enough, it would've caused the dam to float upward. And as the water began to rush out underneath, the enormous pressure would cause the dam to break. Then, by most estimates, about 2500 cubic kilometres of water broke free. The water formed massive waves as it rushed away from Lake Missoula to the west. Along the way, it lifted giant boulders, carved the steep cliffs and rolling hills of Bretz's scablands, and helped shape the vast Columbia River Gorge that today forms the boundary between Washington and Oregon. Pardee eventually wrote up all of this evidence, detailing what happened to the missing lake and connecting it to the floods that Bretz had postulated in 1942.

And in the decades after these two intrepid detectives did their work, other Geologists used newer techniques to establish that these floods actually happened many times, One of the clearest pieces of evidence is in the remains of the bed of Lake Missoula itself. The dark and light bands of sediment on the floor of the lake, known as varves, are like an archive of the years when the lake was full of water. Dark varves correspond to winter deposits and light ones to summer. But some of these layers are interrupted by beds of gravel that was deposited by rapidly moving floodwater. So the number of varves that appear between the layer of gravel tells us that these catastrophic floods happened every 20 to 60 years. And scientists have even been able to track down multiple lines of evidence to estimate when they happened. Over the years, geologists have studied flood deposited in the ocean, where the Columbia River empties into the sea. They've studied the sediments in rocky outcrops and the chemistry of the giant boulders found along the path of the flood. And together these clues suggest that Glacial Lake Missoula flooded many times within a span of 7,000, from around 20,900 to 13,500 years ago.