Lake Missoula and the Great Glacial Flood

Around 10,000 years ago, a huge glacier lake broke through its ice dam and changed much of the landscape in the Pacific Northwest.

US Geological Survey of the Lake Missoula flood zones photo from Wiki Commons

For most of geologic history, North and South America were separated by a stretch of water over 100 miles wide and a mile deep, in what is now Central America. As a result, animals in the two continents took separate evolutionary paths. North America had been part of the northern supercontinent of Laurasia, while South American had been a part of the southern supercontinent of Gondwana. South America shared much of its biology with Australia and Africa, and was dominated by marsupials (mammals with pouches like a Kangaroo) and monotremes (egg-laying mammals like a Platypus). North America, on the other hand, shared wildlife with Europe and Asia, and was dominated by eutherians (“true mammals”, with placentas and live birth).

Then about 3 million years ago, ocean levels dropped and tectonic forces pushed the land up to form the Isthmus of Panama, connecting the two continents. The result was what paleontologists refer to as the Great American Interchange, when wildlife from each continent now had a land route to the other. Deer, horses, camels, tapirs, bears and cats went south, while sloths, armadillos, giant flightless birds and giant rodents went north.

There was also a profound effect on global climates. As the water pathway disappeared, the new land area separated the Caribbean Sea from the Pacific Ocean, and it was no longer possible for warm Caribbean water to be carried up the west coast of North America. Instead, new ocean currents formed which circulated cold water along what is now California, which we know as the Pacific Gyre. This cold water disrupted normal weather patterns and led to colder climate in the north, causing more snowfall which then remained through the cooler summers. Over time, this caused glaciers and ice sheets to form, and in a self-accelerating loop this produced more cold, more snow, and more ice. The ice sheets grew to immense size and advanced slowly over the landscape, reaching as far as the northern United States. The Ice Age had begun. For the next 2.5 million years these ice sheets would periodically expand and retreat, until warming temperatures caused them to finally melt away about 15,000 years ago.

During one of these glacial advances, about 200,000 years ago, a section of the Cordilleran Ice Sheet happened to plug up the Clark Fork River across a valley in Idaho. The ice, over 2000 feet thick, formed a natural dam and caused the river to back up behind it. The immense lake is now known as Lake Missoula. 

Periodically, the ice dam would weaken and break, and Lake Missoula would drain as the newly-freed Clark Fork River was able to run again, only to be plugged up again by another ice dam. This process continued for millennia. Today the remains can be seen in many mountaintops in the northwest, which were at the time tall enough to tower out of the water, and which bear the distinctive parallel geological lines which mark the ancient shorelines as they rose and fell.

By about 15,000 years ago, the global climate was warming and the Ice Age was coming to a close. Lake Missoula at this point was at possibly the largest size it had ever reached. Measuring 3,000 square miles and at least 2,000 feet deep, the lake contained as much fresh water as the modern Great Lakes Ontario and Erie combined.

And then, as the great ice sheets were melting for the last time, the ice dam once again broke, and Lake Missoula drained out in one tremendous rush. It took only days for the huge volume of water to come flooding down the valley and out over the landscape, racing some 400 miles to the Pacific shore. The rushing water killed everything in its path, carried boulders for miles, scoured the landscape of sediments, and carved out channels and new valleys as it flowed. We can still see the aftereffects of the immense flood in areas like the Camas Prairie in Montana. In some low-lying areas, some water remained behind as the rest of Lake Missoula rushed out, and one of these is Flathead Lake, about 50 miles northeast of Camas Prairie. Although Flathead Lake is the largest body of freshwater west of the Missouri River, it is only a tiny fraction of what the glacial Lake Missoula had once been.

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