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Geo-Joint: Lake Missoula

Posted on September 30 2015

Lake Missoula

Most of the processes that have made our landscape happen slowly. Lifting up the Rocky Mountains, carving the Grand Canyon, grinding out Yosemite Valley – that work went on for millions, or at least thousands of years. Sometimes nature randomly sets things up that suddenly burst forth with enormous effect. Explosive volcanoes can do this, or mega-earthquakes, but their major effects are relatively local. This is the story of a big deal with a big footprint.

During the end of the last Ice Age in North America, starting around 20,000 years ago, glaciers would advance and retreat with global temperature changes. During one advance, a lobe of the northern icecap came down and blocked the flow of the Columbia River, as well as the Clark Fork River, a major tributary to the Columbia and the main westward drainage of the Montana region. Since the water couldn’t progress, it ponded behind the glacier, forming Lake Missoula. In time, the glacier damming the river may have stood up to 2000 feet above the waterline, and perhaps 1000 feet below it. The level rose and rose, until the lake covered 3,000 square miles and held 530 cubic miles of water – more than the volume of Lake Erie and Lake Ontario combined. At a certain point, however, the dam of ice began to be floated by the water, and water forcing its way under the glacier dug waterways that rapidly expanded, weakening the barrier.

As the ice plug broke up and the lake was freed, the power unleashed was almost unfathomable. Flowing at perhaps eight to ten cubic miles of water per hour, the pathway it chose was scoured to the bedrock, carrying sand, rock, ice, trees, mammoths, giant beavers and anything else unlucky enough to be in front of the roiling flood. With this flow rate, ten times the combined flow of all the rivers on earth, the entire lake may have emptied in as little as two days. It pushed westward across the panhandle of Idaho, over and across eastern Washington, along the Oregon border, and out the mouth of the Columbia, far into the Pacific. As with many stories of ancient geologic events, researchers dispute the process details. Some feel this gargantuan flood happened not once but multiple times, as the glaciers continuously pushed south, damming the rivers and then suffering breaches over and over through time. There are estimates of 40 or more repeat occurrences.

How did geologists piece the story together? Those scientists in the early and middle part of the 20th century who first postulated a massive flood were mocked for their reliance on a catastrophe to explain the landforms they found. It was felt that all geologic processes must have been slow and unspectacular. But the evidence of epic flooding was too great for some to ignore, and the realization that a massive lake once sat in Montana provided the source. Downstream from it, the landscape told the story. Rolling hills of up to 30 feet in height were actually ripple marks left by massive flooding. Huge boulders deposited in Oregon were made of rock from Montana, floated for hundreds of miles in icebergs. Deposits of sand and gravel found high along the sides of valleys were left behind as waters eased down into valley floors. And perhaps most stark, the so-called channeled scablands of Washington, where vast stretches of bedrock were laid bare. Not far away is farmland soil hundreds of feet thick. The force of the water not only tore away the dirt, it ripped out some of the bedrock as well. By the 1960s, the preponderance of evidence had convinced the majority that this interpretation was right.

There is far more detail to this amazing story – I recommend you look into the geologic history of the Columbia Gorge and its waterfalls, for instance, and the many other fascinating pieces of this tale of water, ice, and gravity and their truly awesome power.


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