The ground trembling came first, Eastern Washington shuddering under the approach of an Ice Age flood of 500 cubic miles of water, weighing more than...

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The ground trembling came first, Eastern Washington shuddering under the approach of an Ice Age flood of 500 cubic miles of water, weighing more than 2 trillion tons.

The sound next, an ominous rumble growing to an overpowering roar. A cloud of mist on the horizon. Beneath it, a towering, unstoppable wave.

The water was a brown slurry, soupy with silt, rocks, trees, icebergs and any animals unlucky enough to get in its path: mammoths, giant sloths, beavers the size of bears. Basalt columns were peeled off like string cheese. Some gravel from Montana would be carried all the way to the Pacific Ocean. Other would be left in bars as high as a 40-story building.

If any humans were in the Northwest then, roughly 15,000 years ago, the inundation would have seemed like the end of the world.

The wave itself was a prow of white water, pushing a shockwave of air. Rivers typically flow from near zero to 7 miles an hour, but this flood started at freeway speeds. In volume, the deluge thundering across the Pacific Northwest was 10 times the combined flow today of all the rivers on Earth.

The flood started in the Idaho panhandle as a wall of water 2,000 feet high, bursting through the remnants of a glacial dam at 65 miles an hour. It spread into temporary lakes as it plowed west and south, and bunched into a rising boil at every canyon and constriction. In the Columbia River Gorge, it rose again as deep as 2,000 feet, its kinetic energy so great that it gouged out a pothole below sea level in the John Day River canyon.

At the Gorge’s western end, the flood depth was still 800 to 1,000 feet, and water shot past Oregon’s Crown Point like a fire hose at speeds as high as 70 to 80 miles an hour. One of its gravel bars would become east Portland, the water there 400 feet deep. The flood backed up the Willamette Valley as far south as Eugene, the swirling current grounding ice chunks that, when melted, deposited odd boulders across future farmland.

A landscape transformed

Roughly 15,000 years ago, Ice Age glaciers blocked swollen rivers and created enormous lakes in western Montana and northeastern Washington. When the glacial dams broke, flood waters rushed across Eastern Washington and the Columbia River Gorge and backed into valleys such as the Yakima and Willamette, helping carve the region’s topography. Geologists estimate these catastrophic floods happened up to 100 times.

On the flood poured, past the future sites of Kalama, Longview and Astoria. Sea level was 300 feet lower then than now, and the coast a hundred miles farther west. The flood crossed this plain and plunged into the sea so violently that it scoured a canyon underwater and carried parts of Montana — 600 million cubic yards of sediment in all — in a curving arc as far south as California.

After a week or two, the flood subsided to normal river levels, the land around stripped bare. Halting recovery began in the ensuing years, vegetation getting a toehold on ravaged floodplains. Then, 30 to 50 years later, another flood would come.

This cataclysm happened as many as 100 times over the next 3,000 years, helping carve the Washington we see today.

Welcome to the Northwest’s proposed newest, most timely tourist attraction.

The predicted effect of today’s global warming in the Northwest is quite grim: more flood, drought and wind, shrinking glaciers and snowpack, rising sea levels, acidic oceans, monster forest fires, invasive species and wholesale collapse and transformation of native ecosystems.

But it still seems like science fiction. Wasn’t it freezing and snowy last winter? Hasn’t the climate been fairly stable for the past 6,000 years in which civilization has risen? Haven’t temperature increases lagged behind atmospheric carbon levels? Do scientists really know the future?

Well, they increasingly know the past. And there’s no better place to contemplate the future effect of our atmosphere’s change than the base of Washington’s Dry Falls at Sun Lakes State Park, a landscape catastrophically carved the last time the planet dramatically warmed. The Ice Age’s end may have been partly due to unexplained “burps” of carbon dioxide from the ocean some 18,000 and 13,000 years ago, according to a paper in the journal Science published in May.

The last Ice Age went out with a roar, not a whimper. The result is one of the most dramatic examples in the world of what climate change can mean, and Congress has plans for a “trail” to commemorate the little-known story.

Signs, interpretive centers and maps following the floods would be coordinated under federal legislation proposed by a Democrat, U.S. Sen. Maria Cantwell of Seattle, and a Republican, U.S. Rep. Doc Hastings of the Tri-Cities. If the bipartisan measure, co-sponsored by much of the rest of the Northwest delegation, passes as expected by the end of this Congress in 2008, the long-sought “Ice Age Floods National Geologic Trail” — similar to the auto route following the path of Lewis and Clark — would be planned by 2010 and in place by 2016, Keith Dunbar of the National Park Service estimates.

“Instead of opening new visitor centers, we’d enhance what’s already been built,” he says.

Federal funding would be a modest $500,000 or so a year to coordinate state and local efforts, plus about $12 million in expected capital spending. “The idea is from the top down but also from the grassroots up,” says Rene Senos, a senior associate at Seattle’s Jones & Jones landscape firm, which has worked on the plan.

Some municipalities aren’t waiting for Congress. The Wenatchee Valley Convention and Visitors Bureau, for example, already has a map of its own 164-mile flood loop, which includes Dry Falls.

Following the entire trail could be a daunting project for the amateur geologist. Its primary roads are estimated to total nearly 1,000 miles long, says Gary Kleinknecht, president of Richland’s Ice Age Flood Institute. Secondary loops could double that.

“You just can’t fit the entire story into a national park,” noted retired Eastern Washington University geologist Eugene Kiver. The only photographs that take in the entire scale of the flood are from space.

A key central point is the geologic marvel of Dry Falls, which is what the name implies — the dry cliffs remaining from an Ice Age waterfall more than twice as high as Niagara and 3 ½ times as wide. It ran when the Ice Age Columbia River flowed through a canyon called Grand Coulee, creating one of the greatest waterfalls in geologic history.

Congressman Hastings has vivid memories of being taken to the site as a boy. Yet as spectacular as Dry Falls was for thousands of years, it was reduced to little more than a hump during the Ice Age floods, so gigantic and overpowering was their wall of water. “It was absolutely massive,” Hastings says.

He likes to visualize the floods from the crest of Rattlesnake Mountain near Hanford. When the water pooled at Wallula Gap, waiting to force its way through the Columbia Gorge, the Tri-Cities site was under 800 feet of water.

The floods carved out Palouse Falls and Eastern Washington’s channeled scablands, sculpted the Columbia River Gorge’s Beacon Rock, and produced the sheer walls on the Oregon side of the Gorge now threaded by lesser cataracts like Multnomah Falls.

For decades, making sense of the story was chiefly an arcane feud among rival geologists, the public only slowly informed. Only in recent years have we realized a distant disaster could be a tourist attraction, enhanced by new curiosity about climate change.

Washington is a geologist’s Disneyland, packing desert, glacier, volcano, sand spit, earthquake and flood into a relatively small area. Two epochs stand out.

The first is the floods of lava that flowed over 63,000 square miles of the Pacific Northwest between 17 million and 11 million years ago, covering the Columbia Basin with basalt up to two miles thick. The surface was flat as pudding until it began to warp and be carved by rivers such as the Columbia.

The second is the most recent Ice Age, from roughly 80,000 to 12,000 years ago. This was just the latest of a series of ice ages, stretching back 2 million years, and that series is in turn just the latest of at least four major ice epochs in our planet’s history, at least one of which turned the globe into a completely frozen “snowball earth.” For this flood story, only the most recent advance of the ice can be traced.

An ice cap covered almost all of Canada and extended down the Puget Sound basin just past Olympia, burying Bellingham under a mile of ice and Seattle under an estimated 3,000 feet.

East of the Cascades, a glacier plugged the present-day course of the Columbia River with a wall of ice near the location of Grand Coulee Dam. The ice was four times higher than the dam’s 550 feet above bedrock. The river backed up to form 1,500-square-mile Glacial Lake Columbia and, blocked from its normal riverbed, spilled south to carve the canyon we call Grand Coulee. Steamboat Rock and Sun Lakes State Park are in the upper and lower coulees today, the upper one partly filled with an irrigation reservoir called Banks Lake.

In Idaho, another ice lobe blocked the path of the Clark Fork River at the site of present-day Lake Pend Oreille. This 2,000-foot-high glacier backed water up in western Montana until it formed Glacial Lake Missoula, totaling 530 cubic miles or more than Lakes Erie and Ontario combined. The Great Salt Lake is the remnant of yet another Ice Age sea that eventually broke through to flood the Snake River Valley.

Just what starts and stops ice ages remains under study, as does the exact sequence that led to the great floods. Solar or astronomical cycles, volcanism and ocean “burps” may all have played a role.

In any event, when Glacial Lake Missoula deepened enough, it may have floated its ice dam and undermined it, the ice giving way in a cracking explosion. Alternately, the ice might have been overtopped. Whatever happened, the huge lake was released all at once.

When water velocity doubles, its kinetic energy quadruples, and the erosive power of the floods was almost unimaginable. Geologists John Elliott Allen and Sam C. Sargent have calculated the combined energy of all the pulses may have been double that of the comet impact 65 million years ago that wiped out the dinosaurs.

Geologist David Alt, in his “Glacial Lake Missoula and its Humungous Floods,” said the deep, fast-moving water formed “kolks,” or extremely powerful whirlpools that plucked at rocks the way tornadoes pluck at mobile homes. Dutch engineers have seen kolks in Europe swirl car-sized boulders like bits of paper. In Eastern Washington, the whirlpools drilled at basalt like jackhammers. Today’s Dry Falls retreated 20 miles from where its lip was first located as successive floods ate backward at the cliff.

Bruce Bjornstad, geologist at Pacific Northwest Laboratories, notes in his “On the Trail of the Ice Age Floods” that the water formed other impressive waterfalls as well, carving the spectacular cliffs of Frenchman Coulee near the Columbia River.

The result is a landscape that puzzled geologists early in the 20th century: what controversial catastrophist J Harlen Bretz called “the channeled scablands.”

Eastern Washington is crisscrossed with dry canyons or ravines called a wash in the Southwest, a wadi in the Middle East, and a coulee here, from the French couler, “to flow.” Bretz, a Seattle high-school biology teacher turned amateur geologist, was puzzled by their proliferation, so he got a geology degree and went exploring. In a 1923 meeting of the Geological Society of America in Seattle, he won a standing ovation for carefully describing the coulees, attributing their presence to glacial runoff.

Two months later, however, he shocked his colleagues by proposing that the coulees had been carved not over eons, but in days or weeks.

The uproar was immediate. For the past 150 years, geology had been combating biblical estimates that Earth was created in 4004 B.C., estimating instead that our planet is actually 750,000 times older: time enough for both erosion and evolution to take place. Now Bretz seemed to be evoking Noah. Worse, he had no idea where the water came from.

Another geologist, Joseph Pardee of the U.S. Geological Survey in Montana, had the answer. In 1909 he realized a vast lake had once submerged present-day Missoula 950 feet deep, formed by a giant finger of ice that pointed down Idaho’s Purcell Valley. He wrote a paper about it. But while geologists castigated the prickly Bretz, he and Pardee never formed an alliance, for reasons historically unclear.

Pardee published his own definitive paper in 1942 giving convincing evidence — ripple marks looming 50 feet high and a hundred yards apart — of a rapid emptying of Lake Missoula in as little as 48 hours.

Still, many geologists remained unconvinced until persuasive field trips in the 1950s, and did not admit their conversion to Bretz until 1962, when he was already retired. One basic geology text wasn’t revised to mention the floods until 1971, and it was only in 1979, when Bretz was 96, that he finally received the Penrose Medal, the nation’s highest geologic award. He died at age 99.

Bretz’s assumption was that one titanic flood was responsible, and Richard Waitt, a geologist now with the Cascades Volcano Observatory in Vancouver, taught just that with colleague Vic Baker in a field trip in 1977. Shortly afterward, however, he spotted a gash carved by an overflowing irrigation canal in the Walla Walla Valley with sediment deposits that could only be explained by multiple giant floods.

“In the field trip we were down to one or two floods,” he recalled. “Two days later, we had 40. That gash proved to be the Rosetta stone.”

Geologist Brian Atwater of the University of Washington later did careful sampling in northeast Washington and counted 89 floods without reaching bottom, leading to present-day estimates of up to 100 catastrophic water releases. The ice lobe would break, fresh ice would flow from Canada to once more dam the Clark Fork, and the cycle would be repeated every 50 years or so. It ended only with the melting of the continental ice cap.

In the past few decades, science has become much more comfortable with catastrophes and the idea that our planet goes through sudden, wrenching changes. One theory is that rising sea levels after the ice age broke through the Bosphorus Straits and rapidly deepened the Black Sea, drowning early civilizations on its shore and giving rise to legends of Noah’s Flood and Atlantis.

Astronomers have found channeled scablands on Mars startlingly similar to Eastern Washington, theorizing this meant that Mars, too, once had titanic floods.

Here on Earth, the result is a proposed trail with an intriguing tale to tell. It includes a geologic mystery with colorful personalities, a story of cataclysmic instead of gradual change, and an easy-to-see example of the consequences of global warming. The route is not just scenic, it is educational and thought-provoking.

Will greenhouse gases be as dramatic this time? While there is no Canadian ice cap now, there are theories that melting of the Greenland Ice Cap could derail the Gulf Stream and ocean circulation, freezing Europe. The collapse of the West Antarctic Ice Sheet could raise sea level 20 feet. Heat energy in the atmosphere could generate more wind and cause more evaporation for rainfall. Africa and the Midwest could become drier, with massive crop failures. But consequences are still debated.

What we do know is that the earth has been gripped in a freeze-thaw cycle for 2 million years, and that civilization has arisen in an inter-glacial period of remarkable climate stability. The cycle of season and temperature that humans have regarded as normal for agriculture may actually be abnormal.

The new trail, then, will serve as both warning and reassurance. The warning is that nature can’t be taken for granted. And the reassurance, like Noah’s rainbow, is that even some of the worst floods in geologic history did, eventually, pass, leaving the Pacific Northwest with the spectacular landscape we have today.

The mammoths are gone, and humans may go, too. Earth, however, abides.

William Dietrich is a Pacific Northwest magazine staff writer. Steve Ringman is a Seattle Times staff photographer.