The Great Missoula Floods

by Bruce Patterson, January 29, 2014

Not many rivers on earth have blasted through mountain ranges. The Bramhaputra River, which rises on the Tibetan Plateau and flows into the Bay of Bengal, cuts through the Himalayas and counts as the planet's most spectacular example. Also, because of its extreme ruggedness and the provincialism of the locals, the Bramhaputra’s gorge was one of the very last places on earth to know the footsteps of outsiders. Eastward, in deep valleys between the paralleling mountain ranges, the Salween River (it flows south through Burma and joins the Indian Ocean just east of Rangoon), the Mekong and the Yangtze also come from Tibet but, like normal rivers, they don’t blast through mountains but outrun them.

Far to the northwest, on the frontier between Europe and Asia, the Ural Mountains stretch as far as the American Rockies and not a single river crosses their divide. Topography is hydrology and, by definition, water isn’t supposed to cross over divides. Nine Siberian river systems empty into the Artic Ocean, four of them are longer than the main stem of the Mississippi, the land is only wrinkled and still not one of them has ever bored through one of its divides.

Until recently it was thought that the Grand Canyon of the Colorado was an example of a river blasting through a mountain range, in this case the forested highlands known as the Kaibab Plateau. When I was a boy the theory of Continental Drift was just gaining acceptance and the science of plate tectonics was just being born. So back then the notion that, over the course of tens, or even hundreds, of millions of years, the “raging river” had carved the gash seemed the only logical explanation. But now we know the opposite is true: the land rose while the river more or less maintained its elevation, washing away, during floods, only that which blocked its path. Down by the river the oldest rock in the cliffs is 1.7 billion years old, but the age of the canyon is 4.5 million years at max.

The Continental US has three Great Divides: the Appalachian (it rises in Alabama and ends at the Potomac River), the Continental that runs from Mexico to Canada and the Pacific Crest that does the same thing. Set them end-to-end and you’ve got well over 3,000 miles of great divide and I know of only three places where water crosses over them. In the Wind River Range of Wyoming there’s a creek (“2-Ocean”) that gushes out of springs on a mountainside and then forks atop a ridge and sends one stream to the Pacific and the other to the Atlantic. In the High Sierra above Mammoth Lakes, the Middle Fork of the San Joaquin River passes through a notch in the glacier-polished granite of the Pacific Crest. I should mention that, north of California’s Mt. Lassen, the Pit, McCloud, Sacramento and Klamath rivers all cross over the Pacific Crest Trail. But that’s because, for a variety of reasons (not the least, the availability of water) the PCT leaves the true divide and instead follows the southern rim of the Siskiyou Plateau westward, then heads north atop the Klamath Mountains, then east atop the Siskiyous before rejoining the Cascades just above the Oregon border. The real divide runs east of the Klamath Lakes and rejoins the main stem of the Cascades (and the PCT) at Mt. Thielsen just north of Crater Lake. Still all four rivers—especially the Klamath—have carved through divides and that makes them special.

Finally, on far grander scale, there’s the gorge the Columbia River has blasted through the heart of the Cascade Mountains. About 75 miles long, U-shaped and between one and two miles wide, the gorge is lined with scoured walls averaging between 1,500 and 3,000 feet in height. The apex of the wall, the north face of Mt. Defiance, rises to 4,960 feet. The walls are laced with 75 waterfalls, the gorge is home to 800 species of wildflowers (18 of which are found no place else) and it draws millions of sightseers every year, the great bulk unaware of the mystery staring them in the face. For while the average annual discharge of water through the gorge at the site of the obliterated divide is 194,000 cubic feet per second (a friend tells me the flow of the Navarro River was recently gauged at 7cfps) it’s impossible that even twice that amount of water could have blasted through such a massive barrier. Yet the river has not just blasted through the mountains but has all but leveled its course along the way, picking up and transporting an estimated 50 cubic miles of highland rock and sediments and depositing them downstream in the coastal lowlands or out into the ocean.

In the early 17th Century European maps of the Pacific Northwest coast showed the mouth of a river called The River of the West. Given its latitude and impressive size, most saw it as the western end of the fabled Northwest Passage until, two centuries later, Lewis and Clark proved otherwise. Forty years on, after leaving Lake Tahoe and exploring the lands north all the way to the Columbia River, Captain John C. Fremont finally killed the myth of the Northwest Passage once and for all. Strangely enough, it was a myth that had its seed in the chronicles left behind by an early 14th Century Venetian merchant and adventurer named Marco Polo.

In 1765, an English seaman named the area flanking the river mouth “Ouragon” after the sound of the native’s name for the river: birth-bark-dish. After nine days of trying, in 1792 the American sea captain Robert Grey broke through the river bar and sailed upstream as far as the mouth of the gorge. He named the river “Columbia” after his ship, the Columbia Redivida (“revival”), which he was sailing around the world.

In 1790 a Spanish cartographer named what we now know as the Cascade Mountains Las Sierras Nevadas de San Antonio (St. Anthony’s Snowy Mountains). Two years later, the English explorer Vancouver named them The Snowy Range. In 1805 Lewis and Clark named them the Western Mountains. About the same time French Canadian fur trappers came upon the gorge and were so impressed they named the river after it: Les Deschutes, or “river of the falls” or, Americanized, “river of rapids” or “cascades.” But the name “Columbia” stuck to the river and somehow the mountain range got named after the rapids and the first upstream tributary became the Deschutes.

For thousands of years the Columbia River Plateau has been home to a wide variety of cultures and all of them had legends to explain not just the gorge, the soaring mountains, volcanoes and glaciers, but the whole world as they knew it. Since the natives believed the natural contains the supernatural, their myths spoke to their relationship to earth. But the Euro-Americans, in the main, saw “soul” (spirituality) as an exclusively human attribute and the rest of the world, living and unliving, as a mere possession and a temporary one at that. In their alienated minds, knowing “god” meant “forsaking” the world. Yet it’s very difficult to forsake something you know so little about (there’s no sacrifice there). Then, least we forget, “god” endowed us with logic, curiosity and imagination for good reason.

Was this marvelous gorge proof positive of the Old Testament’s Deluge? Had Noah sailed overhead in his Ark? Had there once come a flood so powerful it could only have been pushed by the hand of an Almighty God? What else in the Bible could possibly explain this monumental gash washed through the mountains? Others were content to simply dwell in the majesty and mystery, their imaginations searchlights.

About 100 years after the passage of Lewis and Clark, various clues led a Christian geologist named J. Harlan Bretz to what he thought was the solution to the riddle. The large, glacier-polished granite boulders scattered high up on the walls of the gorge provided one clue. How much water would it take to deposit one such weighty specimen some 700 vertical feet above today’s river? More clues lay far to the northeast in the scoured, gorge-scarred dry scrublands the French trappers had named Les coulees (from “to flow”) and Bretz re-named the Channeled Scablands. Hundreds of square miles had clearly been overrun by water—where did the water go? The scablands were also dotted with stranded granite boulders; boulders matching those found in the gorge. Where did the water come from?

After decades of research, in 1922 Bretz submitted a paper for peer review setting forth his claim that one colossal flood—he called it the Spokane Flood—had washed over the region. Hardly anybody believed him, of course. When knowledge is seen as a possession, and possessions are taken as symbols of power, any new idea is greeted with suspicion. It wasn’t until the 1950s that the geological establishment finally conceded that Bretz was onto something, and more decades spent sifting through the evidence before they could finally agree on what had actually happened.

About 15,000 years ago, during the waning days of the last Ice Age, the southeastern corner of the Cordillera Ice Sheet stretched a finger of ice across the lower Clark Fork River. The Clark Fork runs northwest out of the Montana Rockies and settles into Lake Pend up near the top of the Idaho panhandle. As the finger of ice grew to a height of 2,500 feet, it backed up the river and created a lake called Missoula. How much water was in the lake? About 50 cubic miles, or half the volume of Lake Michigan. When, directly behind the ice, the lake reached the depth of 2,000 feet, the finger broke off and the water was released; a wall of water hundreds of feet tall and moving at 80mph—a torrent equaling thirteen times the outflow of the Amazon. It took maybe two days for the lake to empty and, once it was, the ice dam slowly reformed and the process was repeated. Over the course of the next 2,000 years, at least fifteen times ice dams broke and Lake Missoula emptied.

Lake Missoula and its floodpath

Lake Missoula and its flood path

Since no existing watercourse could begin to handle such flows, the floodwaters spread across the flatlands and left behind the dry channels now seen in the rim-rocked arroyos of eastern Washington. The largest of these, the Grand Coulee, stretches for 60 miles. About halfway down its length the coulee is broken by a dry vertical waterfall that is five and a half times the width of Niagara Falls and twice their height. When the waterfall was in full bloom, its volume was ten times the outflow of all of the rivers on earth.

Even though, because of the continental ice sheets, the ocean was much lower and the mouth of what is now the Columbia River was at least 30 miles westward of where it is today, once the deluges had broken through the Cascades they reached the “chokepoint” seen in the topographical wart crowning the northwest corner of the state of Oregon; there where the river bends northward. Because of the chokepoint, the floodwaters backed up the length of the Willamette Valley and created ephemeral lakes whose shorelines can still be seen high up the on hillsides.

2 Responses to The Great Missoula Floods

  1. Reply

    January 29, 2014 at 10:13 am

    This is fascinating stuff. There’s also a very interesting series of ‘Roadside Geology’ books, one of which discusses these processes. It points out the ‘dogleg’ bends in the Columbia between Portland and the coast match almost exactly the displacement between the Cascades and the Sierras (once a continuous cordillera). It seems a good sized portion of Oregon and N. California real estate are migrating North and West in relation to the rest of the continent, motion roughly parallel to the Western portion of the good ol’ San Andreas fault. Yeeha.

  2. Gupatii Reply

    January 29, 2014 at 10:58 am

    I live on the bottom of the former Lake Condon, and have a truck size granite boulder in my yard that came from Montana. Oh, the stories it tells….

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