by David R. Montgomery | Discover Magazine
Washington scablands: Hundreds of square miles of Washington State were gouged by the great Missoula Floods 15,000 years ago.
Geologists long rejected the notion that cataclysmic flood had ever occurred—until one of them found proof of a Noah-like catastrophe in the wildly eroded river valleys of Washington State.
After teaching geology at the University of Washington for a decade, I had become embarrassed that I hadn’t yet seen the deep canyons where tremendous Ice Age floods scoured down into solid rock to sculpt the scablands. So I decided to help lead a field trip for students to see the giant erosion scars on the local landforms.
We drove across the Columbia River and continued eastward, dropping into Moses Coulee, a canyon with vertical walls of layered basalt. We gathered the students on a small rise and asked them how the canyon had formed. They immediately ruled out wind and glaciers. The valley was not U-shaped like a typical glacial valley, and none of us could imagine how wind might gouge a canyon out of hard basalt. But neither were there rivers or streams. After a while I pointed out that we were standing on a pile of gravel. I asked how the rounded granite pebbles came to be there when the closest source of granite lay over the horizon. Silence.
Hiking through eastern Washington canyons littered with exotic boulders is a standard field trip for beginning geologists. It takes a while to register what you see. A dry waterfall hundreds of feet high in the middle of the desert. Giant potholes where no river flows today. Granite boulders parked in a basalt canyon. Gradually the contradictions fall into place and a story unfolds. Where did wayward boulders the size of a car or house come from? What was the source of the water that moved them around and carved the falls? Today, even novice geologists can conjure up eastern Washington’s giant floods.
Long before the discovery of the scablands, geologists dismissed the role of catastrophic floods in interpreting European geology. By the end of the 19th century such ideas not only were out of fashion but were geological heresy. When J Harlen Bretz uncovered evidence of giant floods in eastern Washington in the 1920s, it took most of the 20th century for other geologists to believe him. Geologists had so thoroughly vilified the concept of great floods that they could not believe it when somebody actually found evidence of one.
Bretz was a classic field geologist and a controversial figure throughout his career. In 1925 he presented the story of the region’s giant floods, seeing what others at first could not—and then would not—see. He spent his lifetime piecing together the story of how a raging wall of water hundreds of feet high roared across eastern Washington, carving deep channels before cascading down the Columbia River Gorge as a wall of water high enough to turn Oregon’s Willamette Valley into a vast backwater lake.
Bretz found exotic granite boulders perched on basalt cliffs hundreds of feet above the highest recorded river level. In the scablands, a desolate region stripped of soil, he came across dry waterfalls and potholes hundreds of feet above the modern river. Gigantic gravel bars deposited within dry valleys implied deep, fast-flowing water. Streamlined hills rose like islands, extending more than 100 feet above the scoured-out channelways.
He realized the chaotic landscape had been carved by an enormous flood that chewed deep channels through hundreds of feet of solid basalt. The ancient flood deposited an enormous delta around Portland, Oregon, backing up flow into the Willamette Valley. The waters, he eventually realized, could have come from catastrophic drainage of Lake Missoula, an ancient, glacier-dammed lake in western Montana.
Bretz was ridiculed until 1940, when geologist Joe Pardee described giant ripple marks on the bed of Lake Missoula. The 50-foot-high ripples, he said, were formed by fast-flowing currents and not by the sluggish bottom water of a lake. Only sudden failure of the glacial dam could have released the 2,000-foot-deep lake. The catastrophic release of 600 cubic miles of water through a narrow gap would sweep away everything in its path. In 1979, when Bretz was 97 years old, the Geological Society of America awarded him its highest honor, the Penrose Medal.
Recognition of the Missoula flood helped other geologists identify similar landforms in Asia, Europe, Alaska, and the American Midwest, as well as on Mars. There is now compelling evidence for many gigantic ancient floods where glacial ice dams failed time and again: At the end of the last glaciation, some 10,000 years ago, giant ice-dammed lakes in Eurasia and North America repeatedly produced huge floods. In Siberia, rivers spilled over drainage divides and changed their courses. England’s fate as an island was sealed by erosion from glacial floods that carved the English Channel. These were not global deluges as described in the Genesis story of Noah, but were more focused catastrophic floods taking place throughout the world. They likely inspired stories like Noah’s in many cultures, passed down through generations.