The Ice Dam and Emptying Lake Missoula

The ages of the earth are long, and they change slowly.  To comprehend geologic time requires a mental framework that needs—before all else—a humble acceptance of man and mankind’s brief presence on this planet. 

            But we do have analytic minds and the ability to look, if imperfectly, both forward and backward along the arrow of time.  The time warp I was exposed to this week certainly required a wide-angle lens.  It started with a relaxing cruise on the Shaunodese, a pleasure boat, on Lake Pend Oreille (pronounced Pond-u-ray).   The Pend Orielle is formed by the outflow of the Clark River. It is located in the Purcell Valley of Idaho’s panhandle, only 50 miles from the Canadian border. Isolated beauty defines this region. Now.

            If you can imagine a time 18,000 years ago things would look a bit different.  This would be near the end of our latest (not last, there will be another—and another) Ice Age.  For 2.5 million years combinations of cooler temperatures and increased precipitation had formed continent-wide ice shields that had advanced, retreated and advanced again down North America.  Eventually, by the end of the Wisconsin glaciation, so much water would be sequestered in ice that the oceans would be 300 feet lower than they are today.  [We are, in truth, still rebounding from that last Ice Age.  For example, the floor of Lake Erie is still raising as it “bounces back” from the weight of the glaciers that pushed it down.] In the western half of the continent, the Cordilleran Ice Sheet pushed south, covering all of Puget Sound, norther Washington, Idaho and Montana.  A toe of that glacier pushed into the Purcell Valley where the wall of ice blocked the flow of Clark River.  The river backed up into Montana and created prehistoric Lake Missoula. 

            Missoula was 2000 feet deep and contained more water than Lake Erie and Lake Ontario combined.  Its size is what killed it.  As the volume of water increased, pressure built on the base of the ice dam.  Pressure creates heat and that raised temperatures above the freezing point.  Tiny trickles of water integrated cracks in the ice, increasing friction.  The glacier kept pushing forward, but the footing of the dam now slipped out from under its base.  In a catastrophic tipping point the dam lost its footing, cracked, more water rushed into the cracks, the base gave way and the dam collapsed.  In an instant a wall of mountain-high water crashed onto any land lower than its level. 

            Try to imagine Erie and Ontario emptying themselves in less than a week’s time.  The rush of water scoured the top soil of what we now call the scablands of central Washington.  It carved out the Grand Coulee, flowed through the Columbian River Gorge, dropped erratics (large boulders of non-indigenous material) and destroyed everything in its path. 

            This event did not happen once, but some 40 times during a 2000 year period as the glacier kept blocking the Clark River, creating a lake which then destroys the dam that gave it life.  The cycle ended when the glaciers finally retreated. 

            There were humans alive in the Americas at this time.  They were Asian in their appearance, hunter-gatherers with a clan and tribe structure.  They hunted large mammals like mammoths and lived lives adjusted to the cold.  Most of those people in the path of the flood would have died, but my goodness, what legends have risen from those who survived.  The Athabascan tale is my favorite. 

            The story in the rocks help me keep the faith.