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Northwest Geologic Wonders Forged in Raging Currents of Catastrophic Megafloods

Across Earth's vast and varied landscapes, majestic geologic wonders have been delicately sculpted over time through the tireless, eroding force of water and wind acting upon the planet's rocky surface.

But unlike the slow, steady cascade of flowing water from the mighty Colorado River, which carved away the Grand Canyon's layered walls over the course of millions of years, the barren Channeled Scablands of eastern Washington state were shaped abruptly by the most powerful and colossal freshwater floods in all of Earth's known history.

"It certainly left its mark on eastern Washington," U.S. Geological Survey Research Hydrologist Jim O'Connor said, referring to the state's unique, natural monuments. "It is visible from space."

Many of the numerous cascading torrents were hundreds of feet deep with flows nearly 10 times that of the combined flow of all the world's rivers as they swept across the countryside, shredding apart the Spokane Valley, stripping away the earth and even gouging out pieces of bedrock, according to a USGS report.

"There were quite a few of them," Washington State U.S. Geological Survey Researcher Richard Waitt said.

The surging floodwaters hit a peak discharge of more than 5 billion gallons of water per second, forever altering the land's geology and leaving their 18,000- to 15,000-year-old remnants scattered across thousands of acres of the Washington landscape.

"Scablands is an old farmers' term," Waitt said, describing the barren, rocky regions of eastern Washington stripped of soil by the tremendous floodwaters.

Almost every one of the earth-shattering outburst floods that shaped the Northwest countryside occurred as a result of the catastrophic failure of the glacial ice that was damming former Glacial Lake Missoula.

The glacial lake would refill and drain periodically by releasing nearly 100 megafloods during the last ice age between 18,000 and 15,000 years ago.

The lake was often more than 2,500 feet deep, about twice the depth of Lake Superior, due to a lobe of glacial ice that descended from Canada at the present-day location of Lake Pend Oreille in northern Idaho near Sandpoint. The ice imprisoned part of the Clark Fork Valley and sealed the Clark Fork River at its mouth near the Idaho-Montana border.

Combined with glacial meltwater, snowmelt and rain, the lake would continue to fill, covering nearly 3,000 square miles and containing an estimated 500-cubic miles of water, according to a USGS report.

Traces of Missoula's ancient shores, once buttressed by the Bitterroot Mountains, can be found across portions of western Montana including present-day Missoula, Darby and northward at the south end of Flathead Lake.

During its peak, the massive volume of water accumulated in the lake would undercut the ice dam, Waitt said, likening the process to filling a glass with liquid until the ice floats. As this occurred, the ice would melt and widen the tunnel exponentially releasing the water suddenly.

"Ice dams do not make good dams," O'Connor said.

The tremendous volumes of water released by the lake, which likely drained in a day or two, roared down the Spokane Valley and upon reaching present-day Spokane became constricted, causing spillover into the surrounding valleys, which resulted in the wide braided system of channels known as the scablands.

Regardless of which path the floodwaters went, the raging currents would eventually make their way to the Pasco Basin and across the Columbia Plateau to join the Columbia River before flowing out to sea, Waitt said.

The Dry Falls and Palouse Falls were also forged in the torrents produced by the megafloods. Unlike Niagara Falls, which has slowly receded due to the erosion of water undercutting the base, the falls of the scablands receded much quicker due to the tremendous volume of water, Waitt said.

More evidence of the immense power of the Missoula megafloods are the large ripples, only visible from a bird's eye view, rolling across the Northwest landscape.

"Those ripples are giant current dunes," O'Connor said. "They're the same thing you would see in a stream except they are made of sand, gravel and boulders."

Waitt said the ripples are the same principal of the ripples you would see in a river but are colossal in comparison due to the awesome force and water volumes released by the Missoula megafloods.

During the last ice age, the North American landscape and climate was far different as huge lobes of glacial ice descended into the valleys of the Northwest mountain ranges.

"It was much colder and wetter," Waitt said.

While no evidence exists to indicate humans were living in the vicinity at the time, according to Waitt, it is possible early North American people could have witnessed these apocalyptic, ancient floods.

The present-day site of the Great Salt Lake basin in Utah also once housed an incredible volume of water which spawned another destructive, North American megaflood, occurring around the same geologic time of the Missoula floods.

However, the peak discharge of former Lake Bonneville, which covered much of Utah and parts of Idaho and Nevada, was small in comparison to Missoula's outburst floods and took longer to drain.

The present-day Great Salt Lake is a remnant of Bonneville, O'Connor said.

Lake Bonneville began to fill when the region was much wetter and colder, he added. In time, the lake began to fill until its walls could no longer hold the massive volume of water.

"It is just like a bathtub that is plugged," he said.

The Bonneville megaflood hit a peak discharge of about 1 million cubic meters per second, far less than the Missoula floods of 20 million cubic meters per second, but the volume of water surging into the Snake River Plain and joining Snake River was nearly double.

The floodwaters drained into portions of southern Idaho, and in eastern Washington, where flood evidence of both Missoula floods and the Bonneville floodwaters converge, Waitt said.

The Bonneville floodwaters also flowed into the Columbia River before making their way out to sea.

While the scale and tremendous volumes of water released by these ice-age floods rival any contemporary flood in human history, outburst floods from smaller glaciers of the kind seen today can and still occur, Meteorologist Jim Andrews said.

Recent outburst floods due to ice-dam failures continue to occur in Alaska near the Hubbard Glacier and the Mendenhall Glacier.

"Meltwater lakes are trapped against or behind ice edges, leading to sudden release of pent-up water," Andrews said.