The Great Lake Missoula Floods

When I first heard about the Channeled Scablands of eastern Washington over 40 years ago in Dr. Gerry Johnson's Historical Geology class at the College of William and Mary I was mesmerized. The   prospect of an unimaginably huge flood caused by the failure of an ice dam that formed Glacial Lake Missoula was almost too much to believe. A wall of water hundreds, maybe a thousand, feet high moving house-sized chunks of rocks and Lord knows what else.

Even where I live now - western Oregon's Willamette Valley - was not spared the wrath of the raging waters unleashed by the dam's failure.

Almost too much for a 20-year-old from Long Island to fathom!

Lake Missoula was larger (volume-wise)  than Lakes Erie and Ontario combined - 550 cubic miles versus about 512 cubic miles; the ice dam forming the lake was about 2000 feet high. The dam was part of the Cordilleran Ice Sheet near the end of the Pleistocene Epoch, about 15,000 - 18,000 years ago. It blocked what is now the Clark Fork River to form the lake.

At the time I learned all this (c. 1969) we thought it had happened just once. But there's now strong evidence suggesting that Lake Missoula emptied and filled dozens of times.

Here is a good description of the Channeled Scablands with some excellent pictures, and an very good verbal description of the flood from the Montana Natural History Center. And here is a more technical discussion from the USGS.

Early geologists recognized the unusual aspects of the scablands but could not explain them. The proposal that the scablands were produced by a catastrophic flood was not greeted with enthusiasm by geologists of the early 20th century.

From the Montana Natural History Center:

Two geologists, J. Harlen Bretz and Joseph T. Pardee, were instrumental in finding the solution to this geologic mystery. Bretz spent a great deal of his life studying the geologic landscape of eastern Washington. He dubbed the scarred landscape "The Channeled Scablands", and in 1923 he began to publish a series of papers on the Channeled Scablands of Eastern Washington. Bretz realized that only moving water could have formed the features in eastern Washington. Bretz proposed what many people thought was an outrageous hypothesis; these features must have been formed by large scale flooding of catastrophic proportion.

The geologic community received the idea of a catastrophic flood poorly. At this time, most geologists abided by the principles of Uniformitarianism, [as opposed to Catastrophism] the idea that past geological events can be explained by forces observable today. Since a flood of that proportion had never been seen, Bretz's idea was quickly dismissed. To make matters worse, Bretz could not identify the source of this catastrophic flood. The controversy ensued until 1942 when Joseph T. Pardee introduced new evidence suggesting a possible source for the catastrophic flood.

So why am I 'channeling' my early days as a budding geologist?

Just a few days ago, The Oregonian carried a wonderful story about a first: the USGS's computer simulation of one particular megaflood that occurred between 15,000 and 18,000 years BP. The graphic below is byEric Baker of The Oregonian and shows four snapshots over the course of the draining of Lake Missoula: 550 cubic miles of water in 55 hours!

Maximum discharge was about 1.3 billion gallons per second, about 1,000 times the Columbia River's current average flow, with water levels as much as about 1,000 feet above the river level when the flood entered the Columbia River Gorge. When it arrived at the current site of Portland, OR, the flood was still about 400 feet above the normal river stage.

 
Questions still remain, however. Joe Rojas-Burke reports:

But controversy persists. A few scientists assert that the cataclysmic floods must have had multiple sources, not just an outburst from Lake Missoula. John Shaw of the University of Alberta in Edmonton, for instance, has proposed that an enormous reservoir beneath the ice sheet over much of central British Columbia boosted the flooding.

The new simulation suggests that discharge from Lake Missoula alone would have been powerful enough. The simulated flood reaches peak stages all along its route that match the evidence visible today in sediment, with one big exception: At Wallula Gap, water levels in the simulation fell short by as much as 130 feet.

"It's pretty clear, if Lake Missoula is enough to hit all the other high water marks, you don't need another source of water," Denlinger [Roger Denlinger of the USGS] said. Calculating the convoluted paths of such a massive flood requires an immense amount of number crunching. Simulating one flood requires more than 8 months of computer time, Denlinger said.

But the computer simulation isn't likely to end the debate. The fact that it can't reproduce the maximum flooding at Wallula Gap leaves room for doubts. And some experts say there is direct evidence for an additional source of flood waters from beneath the ice sheet that covered the Okanagan Valley [in Canada].

"It is conceivable that other valleys in southern British Columbia contributed water to the scablands but the field evidence necessary to test these possibilities has not been fully documented," said earth scientist Jerome-Etienne Lesemann at the University of Aarhus in Denmark.

"There are a number of unanswered questions," he said. "That makes the whole Channeled Scablands story a really interesting and intriguing geological puzzle."

Hydrology gone wild! Amazing!

"Civilization exists by geological consent, subject to change without notice." -- Will Durant

Comments

[cody miles]

cool

[Michael]

Hi, Donn.

You are confusing me with someone else. I was not in Edmonds on 1 November nor do I have a video of the Missoula Floods.

[Donn Charmley]

How can I obtain a copy of the 'video' on the Missoula Floods you showed in Edmonds on Nov.1st?

[George]

I spent the first 12 years of my life in Missoula. We lived near the university just below a fair-sized mountain. Sometimes we'd hike to the top and explore caves and such, and wondered why there were so many sea shells everywhere. Even before we were educated in such things, we speculated that everything around us must have been underwater long ago...

[Michael]

Hi, John.

Thanks for commenting.

Yes, it was wonderful - the event, that is!
You would have been obliterated.

I would love to see a demo of the simulations.

What was amazing was that the session on the Lake Missoula floods fomented a weeks-long discussion on 'catastrophism' vs. 'gradualism' and 'uniformitarianism.' And of course, this was long before concrete evidence of the meteorite impacts although they had been suggested by that time.

And Gerry Johnson was such a great teacher he might have made a paleontologist or a stratigrapher out of me!

[John Fleck]

Wonderful! As a college student, I too was mesmerized. And it looks from the map that the classroom I was sitting in (at Whitman College in Walla Walla) would have been underwater.