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Monday, May 04, 2015

Geological Cataclysm - The Missoula Floods

Western Washington state has a very interesting, if barren, landscape.  It took a fascinating geological detective story to understand how it got this way.  What is amazing is how awesome and violent the process was that produced this landscape!

The image below shows an area known as the Channeled Scablands.  Spokane, Washington is the city at the upper right.




Notice in the satellite image how there are dark areas of exposed bedrock, seemingly like enormous drainage patterns?  That's exactly what it is, but it is vast in scale - much of the state is stripped down to bedrock.  What happened here???

Below is how the scablands look from ground level.  Certainly looks like water carved this canyon, doesn't it?

You will notice there isn't a huge river that might have carved such deep canyons over such a wide area.  Within the soft sandstone walls of the Grand Canyon, we can still see the culprit (the Colorado River), but in the scablands we don't find the quantity of water necessary to cut so deeply into solid basalt, which is a very, very hard type of rock.

In 1923 a geologist named J. Harlan Bretz, who had been studying the scablands, published a paper arguing that they had been created very suddenly, by massive floods.  His theory was met with skepticism and hostility, even though nobody could offer a better theory.  Bretz was not able to show evidence for the source of the floodwaters, although he was quite certain that his theory was correct.

The reason for the skepticism is that geologists have a belief system called gradualism, and it dominated their thinking at the time.  The assumption of gradualism is that geological processes occur over millions of years.  In truth, most geological processes do occur over millions of years.  Pompeii and Krakatoa (which they knew about), however, happened over mere hours.

Another geologist, J.T. Pardee, had been gathering evidence of a massive Ice Age lake.  He thought that it was a static lake, never realizing that it might have suddenly drained.  Working together over 30 years, Pardee and Bretz gathered evidence to prove their theory of the lake as a possible source for the water that made the scablands.  In the end, their theory won over the gradualists.  The Missoula floods are now sometimes called the Bretz floods.

Here is what geologists now believe happened:  About 17 million years ago the hot-spot of magma that now sits under Yellowstone National Park was underneath Eastern Oregon.  It cracked through the crust and flooded much of the Northwest with a deep layer of basalt, a very dense and hard black rock.  Over the next 17 million years this massive and deep layer of basalt was covered by other material (topsoil), but the basalt did not suffer much erosion.

There is a river that originates in Montana called the Clark Fork river.  The river basin includes most of western Montana.  The river flows northwest out of Montana, cuts through the Bitterroot Mountain range, and then enters Lake Pend O'Reille in Northern Idaho.

17,000 years ago, the last Ice Age was upon the earth, and a large, very deep ice sheet covered all of British Columbia.  This massive ice sheet was advancing into northern parts of the US.  The ice sheet is called the Cordilleran sheet, after a mountainous region in Canada that was carved by it.

The Cordillearan Ice sheet had three lobes (or fingers) that advanced further south than the main ice sheet, and those are the:

  • Puget Lobe - responsible for carving the Puget sound area between Vancouver, Canada and Olympia, Washington
  • The Okanogan Lobe - which blocked the flow of the Columbia River and forced it into its current location at Grand Coulee
  • The Purcell Lobe - which was responsible for floods almost beyond imagination.

The Purcell Lobe of the Cordilleran ice sheet worked its way far enough south to block the flow of the Clark Fork river where it cuts through the Bitterroot mountains at the Idaho-Montana border.  And although the earth was gripped in an Ice Age, the Clark Fork river originated to the south, and so was not frozen solid.  The glacier blocked the flow of the river and backed it up, forming a lake.  A really, really big lake, named Glacial Lake Missoula.

Below, Glacial Lake Missoula to the right of the ice dam.


The height of the ice dam is thought to have been 2500 ft, so it would have been able to hold back a lot of water - about 500 cubic miles of it.  Eventually however, the water broke the ice dam - either from pressure, undermining it, or by simply floating the ice when the lake became deep enough.

Below is a hillside in Missoula, Montana, showing the glacial shorelines.



Whatever the failure mechanism, the ice dam broke.  A volume of water equal to half of Lake Michigan suddenly poured out across Idaho, Washington, and Oregon.  Estimates place the flow at 10 times the combined flow of all the rivers in the world.

This unbelievable volume of water stripped away the topsoil and clawed deep gouges in the basalt bedrock - not over millions of years, but over a handful of weeks.  In addition to topsoil, the floods also carried along enormous chunks of the broken ice dam.

The flow of water from the flood left giant ripples across the landscape from the emptying of Glacial Lake Missoula.  Some are 30 feet tall, like these on the Camas Prairie (note the big rig at right):

Ripples on a plateu well above the Columbia River, west of Quincy Washington.  The river is wide and high at this point because it is held back by the Wanapum dam.  The floodwaters were much higher than the modern pool behind the dam.


The floodwaters burst forth across the Western US in a 400 ft high wall of water, moving at a speed of about 70 miles/hr.  The water reached a choke point called the Wallula Gap on the Columbia River, backing up and flooding the tributary Snake River backwards clear to Lewiston Idaho.

The next choke point was the Columbia River Gorge, where the Columbia river cuts through the Cascade range.  This backed the flood up over much of central Washington.  The final choke point before the water reached the Pacific Ocean was the narrows near Kalama, Washington.  This backed up the floods into the Willamette valley in Oregon, flooding what is now Eugene.  This particular backwater allowed much of the topsoil stripped from Washington to settle.  This deposited topsoil is what makes the Willamette valley some of the best farmland anywhere.

It's a pretty big flood when one third of a large state is suddenly swept away or underwater...

Previously I mentioned that large chunks of the ice dam were carried along with the torrent of water.  Glaciers have a tendency to move large amounts of rock along with them as they grind along the ground.  Sometimes the stones are encapsulated in the glacier.  The bigger the glacier, the bigger rocks it can move.  These rocks are called "erratics", because the glacier deposited them far from where they originated.  Let's take a look:

This rock is an erratic that was rafted in an enormous ice chunk from Montana to McMinnville, Oregon, the eddy where the ice came to rest and eventually melted, depositing the boulder.

Another erratic.  A granite boulder deposited in a basalt canyon.

Even more amazing than this catastrophic flood, is that it now appears that there was not just a single flood, but multiple gigantic floods, as the ice dam was able to re-form after Glacial Lake Missoula had emptied out.

Below:  Burlingame canyon, near Walla Walla, Washington.  The canyon walls show what are believed to be multiple deposits of mud from a series of catastrophic floods, from glacial lake Missoula.




Time now for an awesome video about the megaflood!

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