Mile-High Ice Dams, Glacial Superlakes, Prehistoric Megafloods, and "Grand Coulees"

What happens to an ice cube when you drop it into a glass of water?  That’s easy!  It floats to the top, right?  What happens when you pond several rivers behind an ice dam half a mile high?  If the water gets deep enough behind the dam, the ice will float, right? 

Now you have a picture of what happened in eastern Washington many times during the Pleistocene ice ages.  Geologists have estimated at least 41 separate gigantic floods, the world’s largest ever, rushed across the Columbia Plateau Basalts of eastern Washington when the ice dam broke.  The continental glacier in British Columbia was called the Cordilleran Ice Sheet, which continued to push lobes of ice across Washington, Idaho, and Montana throughout this period.  These lobes pushed the Columbia River south to its present channel.  When the river channels were blocked, the lake backed up behind each ice lobe was called Lake Missoula.  This lake filled the entire valleys of Coeur d’Alene Lake, Lake Pend Oreille, and Flathead Lake.  The floods that occurred each time the ice lobe floated were called the Spokane Floods.

When each dam broke, these immense floods were apparently as much as 600 feet deep at places like Spokane and Lewiston, Idaho, for days at a time.  The amount of erosion that can be caused by a 600-foot high wall of water is simply unfathomable in today’s world.  Where the floods were confined, they dug deep channels, called “coulees,” into the basalt that are now dry.  My June 25 photo below shows Grand Coulee, which now carries the Columbia River, but which was originally carved into the Columbia Plateau Basalts and underlying granite by the Spokane Floods. 

 

Where the flood waters spread out, they scoured all of the soil, loess (windblown silt), loose rock, and vegetation off vast areas of the basalt lava flow, leaving hils and valleys of exposed basalt.  These stripped areas can be seen along US 2 just east of the town of Creston and along I-90 between Sprague and Four Lakes.  The following satellite image shows the huge scoured areas that are now called the “channeled scablands.”

 

However, these floods were not merely destructive, they were also constructive.  When the flood waters receeded, they left behind enormous volumes of gravel, sand, and silt.  The next photo shows some nearly level sand and gravel terraces as much as 200-feet high formed by receeding Spokane floodwater in Grand Coulee.  Each terrace represents one recession event, with successive floods partially eroding the last terrace and redpositing that material downstream.

 

Another type of depostion in northern Idaho and eastern Washington during the ice ages was not a product of the Spokane Floods, but of winds.  The amount of basalt rock ground up into rock flour by the glaciers during the ice ages was immense.  As the glaciers receeded, this silt-sizes material was picked up by the wind and deposited in “dust dunes.”  After the ice ages, these gently rolling hills were vegetated by grasses, but few trees that we now call the “Palouse.”  Today, these hills form some of the richest wheat-farming regions of the world.  The photo below shows an example of Palouse terrain along US 2 near Creston, WA.

Next time: We finally meet our Milepost  “Central Approach Route” to the Alaska Highway with a discussion of the geology of the “exotic terrane” along US/BC 97 in the Okanogan-Okanagan valley.