Wednesday, June 13, 2007

When Da Levee Bruk

Imagine a wall of ice nearly half a mile high. Glacial ice: sere and deep blue, the blue of a newborn’s eyes, or of a serene summer sky. Ice formed in the high, cold plateau of the Canadian Arctic from snows a thousand winters deep. Ice that has flowed and crept and ground its’ way down from the north to close the gap across the canyon of the Clark Fork River. An ice wall, a dam, across the river, now fat and swollen glacial blue with the meltwater of that great ice sheet. An ice dam that now impounds an inland sea; 1,000 feet deep, 200 miles long and dozen of miles wide. 500 cubic miles of water.

500 cubic miles. Cubic. Fucking. Miles.

Of water.

Glacial Lake Missoula.

Imagine, then, the pressure at the base of the dam. The ice wall, seamed and fissured by pressure, by the stresses of traveling thousands of miles from the snowy field of its origin. And the water, beneath tens of millions of gallons of water above it, forcing its way into the cracks, wedging into those fissures. Fingers of water driven by incalculable mass; melting, shearing, driving deeper and deeper into the glacier. Wedging the frozen mass away from the bare rock on which it rests. Burrowing ever closer to the far side of the monster jumble of ice, snow and rock. Closer…closer…

And then, finally, a jet of water from the base of the ice dam. Another. Three more, a dozen; the first block tumbles from the bulging pile. Groaning, shrieking, the ice wall collapses, the water boiling through.

Another flood has begun.
I’ll turn the blog over to a couple of experts. First, the National Park Service:

“When Glacial Lake Missoula burst through the ice dam and exploded
downstream, it did so at a rate 10 times the combined flow of all the rivers of the world. This towering mass of water and ice literally shook the ground as it thundered towards the Pacific Ocean, stripping away thick soils and cutting deep canyons in the underlying bedrock. With flood waters roaring across the landscape at speeds approaching 65 miles per hour, the lake would have drained in as little as 48 hours.”

Now here’s Dick Waitt, from his brilliant 1985 paper on the floods as "jokulhlaups", glacial outburst floods:

“From stratigraphic successions of approximately 40 rhythmic beds at exposures in Montana, Washington, and Oregon, Waitt (1980b) inferred that approximately 40 great jökulhlaups had escaped last glacial Lake Missoula. The number "40" is a minimum; there were at least that many huge floods during the last glaciation. Although problems remain on the number and correlation of events attributed in various areas to successive Missoula floods, regionally scattered sections indicate that there were more than 40 colossal last-glacial floods, probably more than 60. “

Thanks, guys. I'll take it from here.

Of all the geologic acts that shaped my home here in the Pacific Northwest, perhaps the most dramatic and greatest were these: the Missoula Floods. Everywhere I go, every day, I see the work done by the last of these inundations. Even our little house, way up in North Portland, is set on the tail end of the monster pendant bar of Alameda Ridge, a pile of boulders, gravel sand and silt that settled in the calm water behind the volcano we now call Rocky Butte, the then-conical hill serving as a flowbreak and being ripped to pieces in the process.
The floods ripped channels, tore down hills, dumped tons of everything from boulders the size of cars to acres of silt and clay across the entire Willamette Valley. The floodwaters, contained by restrictions in the Columbia River channel further west, formed a lake that backed up into the valley south of Salem and beyond. This “Lake Allison” must have been an incredible sight for decades after each flood, stretching from Coast Range to Cascades and full of everything from tree trunks to icebergs, like the one that deposited this “glacial erratic” on a hillside above the little town of Sheridan, Oregon.

I am always amazed at just how incredibly cool geology and the way the world works are.

One of the questions I was always asked by my students during our PCC geology field trips was: “Were there any people who could have actually seen the floods?” As of today no fossil remains have been found in any of the flood deposits, although we know there were many large animals living on the Columbia Plateau at the time. When Harlan Bretz’s flood theories were accepted our notion of human arrival in North America postdated the last flood by about 1,000 years. Those ideas are in the process of changing. So it is possible, and even likely, that small bands of firstcomers were abroad on the high plateau at the time.
Picture that moment, now part of our Pleistocene prehistory. The night is cold, but the nights are always cold. You aren’t really aware of the chill, just as you’re not really aware of the hunger ache in your belly, or sore feet from the day’s hunt. They’re a part of you, like the ice-bright stars above your head in the depthless velvet sky. You’re tired, but for some reason you can’t sleep. You stretch propped up on your elbows amid the little group sleeping around the dying fire.

Maybe it’s a tiny tremor in the ground beneath you. Maybe it’s a sound, a breathless hush of a sound, just below the threshold of true hearing. Maybe it’s just a feeling inside you, like the feeling you’ve learned to trust while hunting, the feeling that something has changed. You sit up, then you stand up. You stare out into the darkness, willing your eyes to see whatever it is that is pressing on you like a stone on your chest. You know, now, you know that something is out beyond the ring of firelight, something moving, something vast, something frightening, but you can’t see and don’t know what it is.

Perhaps you have time to wake your companions. Perhaps you all jump up, try to run, try to hide, dissolve into hopeless panic before the wall of water sweeps you all away into chaos and the darkness of the vanished years.

Or perhaps you can only stand there, transfixed, staring at the long white line across the black horizon, as the first cold puff of wind racing out before the waters shivers your cheek like a lover’s last caress.


walternatives said...

"500 cubic miles" and "...10 times the combined flow of all the rivers in the world.." Holy shit! Literally, the skin on my legs was awash in chill bumps at the post's end - such vivid imagery. Thank you for this. Learning something so monumental AND liking the learning. Bravo!

atomic mama said...

This is one of the most amazing events ever, and I am so thrilled you've told it! I learned about it in my Quaternary geology course in grad. school and never forgot it. F'ing CRAZY. I LOVE the Pleistocene (especially the last parts).

Talyssa said...

That was really an amazing read! I'm sitting here wondering how you discover all these things to write about!