THE RISE OF GLACIAL LAKE MISSOULA
AT THE START—About 65 million years ago some of the planet's oldest rocks, the Belt Supergroup (some 1.4 billion years old) were crumpled and sliced creating western Montana's mountains and valleys. The Pacific Ocean's coast line at that time was about on the Montana/Idaho border.
Then about 2.6 million years ago at the start of the Pleistocene Epoch the ice age began. Western Montana became wetter and cooler. At least four, and possibly as many as twenty, glacial advances and retreats or interglacial times occurred from that time until 30,000 years ago when the Pinedale glaciation started. It was during this period that Glacial Lake Missoula was created.
Glacial Lake Missoula Forms—About 15,000 years ago a glacial ice sheet a few thousand feet thick advanced into the northern edge of western Montana. It was a finger from this ice sheet that pushed down the Purcell Trench in northern Idaho, near Lake Pend Oreille, that dammed up the Clark Fork River. This ice dam created Glacial Lake Missoula.
The water of Glacial Lake Missoula flooded western Montana's valleys from Heron on the west, to Darby on the south, to Ovando and Garrison on the east, and to Hot Springs and Polson on the north. Even Evaro Hill was at times under water.
In Sanders County all the present town sites had about 1,500 to 2,000 feet of water over them, Missoula 1,000 feet, and Hamilton 600 feet. The surface area of this gigantic lake was about 3,000 square miles. It was as big as present day Lake Erie and Lake Ontario combined, and contained about 500-600 cubic miles of water and was basically an inland sea.
Glacial Lake Missoula would fill and the ice dam would fail, then more ice from the Cordilleran ice sheet would move down the Purcell Trench to block the river, filling Glacial Lake Missoula again and again for an estimated 36 times. The first filling took the longest, 58 years or so, and the last filling the shortest, about nine years. Glacial Lake Missoula's history of filling and emptying lasted about 1,000 years. These events unleashed the greatest floods of known geologic record anywhere in the world.
The Ice Dam Fails—The water at the ice dam had reached 2,000 feet deep about 13,000 years ago. The water at the bottom of the ice dam was under tremendous pressure. This pressure forced its way into any crack, fissure or fracture that developed in the ice, enlarged them and undermined the integrity of the ice dam. Since ice is 10 percent lighter than an equal volume of water, when the water rose and reached 90 percent of the height of the ice dam; the ice began to float (like ice in a glass of water). Between the pressure and the rising water, the ice dam eventually failed, catastrophically. Some geologists think it was the most awesome event of the entire Pleistocene.
A mile wide breach in the dam unleashed 500-600 cubic miles of water that had been backed up for hundreds of miles. The water, weighing 2.5 trillion tons, traveled from the mountain valleys of western Montana to the Pacific Ocean. From Montana it flowed out over northern Idaho, into eastern Washington (creating the Channeled Scablands), then down the Columbia Gorge into Oregon, backed up into the Willamette Valley as far as Eugene, and then out into the Pacific Ocean scouring out the ocean floor. Glacial Lake Missoula's water even flowed up the Snake River past the Salmon River and up the Clearwater River to Kamiah, Idaho (because it was bottlenecked at Wallula Gap).
This flood carved out over 50 cubic miles of earth, widened and deepened the Columbia Gorge, piled mountains of gravel 30 stories tall, carved out Grand Coulee, Palouse Falls and Dry Falls and swept away about 3,000 square miles of the Columbia Plateau and "channeled" the loess hills of the Palouse. It dropped boulders the size of small buses all along its path. During the flood Spokane, Washington was under 500 feet of water, and Portland, Oregon and the Willamette Valley, under 400 feet. The fertile soil in the Willamette came from Glacial Lake Missoula's floods.
Glacial Lake Missoula's sudden "emptying" also scoured the sides of many canyons in western Montana, formed kolks (vertical underwater tornadoes created from deep, fast moving water) that bored out some of our Belt Supergroup bedrock, some of the hardest rock in the world. The fast water and resulting kolks formed Rainbow Lake, Banana Lakes, Toolman Slough, and Duck Pond, Markle, Wilks Creek and Big Creek passes and more. At Rainbow Lake the flood ripped out bedrock several hundred feet deep for five miles and deposited it into the Boyer Bench and northwest of Locust Hill just north of Plains, Montana.
It is said that if you double the velocity of a river, it can carry 64 times more rocks and sediments. The Clark Fork River at twenty times its normal speed, via Glacial Lake Missoula's emptying, carried millions upon millions times more rocks and sediments than normal. It's hard to imagine or even visualize the enormousness of Glacial Lake Missoula, even from an airplane.
Kolks at Markle, Duck Pond, Wilks Creek and Big Creek passes "excavated" 2.6 billion cubic feet of rock. The water at Camas Prairie during the flood was up to 800 feet deep and traveling at 55 mph. This fast outpouring of water created ripples on the Camas Prairie that are 20 to 30 feet high, 200 to 500 feet apart, and up to approximately one mile long. These ripples are among the largest in the world.
The flood would have had a sound and wind ahead of this 500 to 1000 foot high mass of water, ice, boulders and gravels (at speeds of 25-60 mph) that could be heard and felt 30 minutes before it reached you. It has been said that it would have been 100 to 1000 times louder than anything you have ever heard. Once the dam failed, the water took only a few hours to reach the Pacific Ocean and the entire lake emptied in hours to a few days. It flowed 10 times more water than all of the rivers of the world combined. It had a flow rate, just east of Thompson Falls at the Eddy Narrows, at 386 million cubic feet per second at about 60 mph (the Mississippi's big flood of 1993 flowed 1 million cubic feet per second and the Amazon's average flow is about 250,000 cubic feet per second).
The Ice Age Floods Institute (IAFI) at IAFI.ORG has a large amount of information about Glacial Lake Missoula floods. The Institute has books, videos and maps available for purchase. You may also become a member of the Ice Age Floods Institute, and when you do, you may also join one of the IAFI Chapters located throughout the hugh-floods area.
If you have an interest in learning more about the ice age floods, IAFI conducts field trips and presentations throughout the year. Visit the IAFI Calendar of Events at Calendar of Events for more information.