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Understanding the Extent of Glacial Lake Missoula

For tens of thousands of years, glacial ice from ice ages advanced, retreated, advanced, and retreated in a long-term repeating cycle. At times, a mile or more of ice covered much of North America. During one of those periods, Glacial Lake Missoula was formed and the extent of the lake is difficult to really wrap our minds around.

The lake formed when a wall of ice, a glacier, flowed roughly south from what is now Canada and blocked the Clark Fork River. That glacier ice was at least 2,000 feet high; between a quarter-mile and a half-mile. Without a good steady outlet, the lake filled up with water. It wasn’t a fast process, but then, glaciers are usually very slow moving things and 2,000 feet of ice is a lot of ice. 

The Clark Fork River drainage is the most important for the western side of Montana, west of the Continental Divide, and a great deal of water flows down the river. However, it still took centuries for the lake to fill up. It covered about 3,000 square miles when it was full and it held an estimated 500 to 600 cubic miles of water. 

A cubic mile of water is a mile wide, a mile long, and a mile deep. It is quite difficult to imagine this amount of water. It is over half the volume of water that is in Lake Michigan. One cubic mile of water contains about 1,100,000,000,000 or 1.1 trillion gallons of water and it weighs about 9,200,000,000,000 lbs – 9.2 trillion pounds or 4.6 billion tons.

To get at least a bit of a notion of the extent of the lake, the above picture was taken on what would have been the shoreline of Lake Missoula, on what would have been an island in the middle of the lake. Everything in the picture to the distant mountains would have been underwater.

Liquid water is one of the things that makes the earth unique and it is required for life. However, water is also the most destructive force in nature. Dozens of times over thousands of years, the water in Glacial Lake Missoula eroded its way through the ice and flooded to the Pacific Ocean. These could be considered minor floods, though the water volume was great. Each time, once the pressure was relieved, the ice dam would rebuild itself and the lake would fill up again.

Finally, after centuries of erosion of the ice wall and subsequent small floods, each of which released more water than occurs during a major flood of the Mississippi, the ice dam broke entirely, roughly 13,000 to 17,000 years ago. This wasn’t a minor breach like the other floods had been. The ice dam completely collapsed.

There was no longer anything holding the water back, yet geologists figure that it took between weeks and years for all of the water to drain because the volume was so enormous.

Put into perspective, the amount of water that drains through the Amazon River is about a fifth of all the water discharged by all the rivers in the world, combined. In fact, more water drains through the Amazon River than through the next seven largest rivers in the world, all together. Yet, when the ice dam that held Glacial Lake Missoula burst, the amount of water that flowed from the lake represents the amount of water that would drain from 60 Amazon Rivers. 

This map (USGS CC0 shows where the flood waters went (the yellow and orange areas in the map):

The raw power of all of that water scouring the land was unimaginable. The water took enormous chunks of the mountains with it, blasting away solid rock more easily than a construction crew could do it using dynamite. The uncontainable water flow carried boulders that were bigger than cars. Some of these huge rocks were carried as far as 500 miles.

Along the way, the flood waters left sediment, which formed land ripples that were 20-40 feet high or higher and which are still quite visible today. The lake also left marks on hills and mountainsides, like this:

This is an aerial picture of part of the city of Missoula. The hills that are shown just on the other side of Missoula would have been on the shoreline of Glacial Lake Missoula. Especially notice the horizontal bands on the hillside beyond Missoula. Those lines were created by Lake Missoula. Incidentally, the river that can be seen in this picture is the Clark Fork River.

Here is a picture was taken by the Bureau of Land Management in Oregon. Notice the same horizontal banding in the rocks. These were caused by the passage of the flood waters. That gives a notion of the depth of the floodwaters, even as far away as Oregon.

Below is yet another picture that shows the passage of the glacial floods. The hills in the middle of this picture aren’t hills at all, they are the ripples of land caused by sediments being deposited as the flood waters passed. This image was taken in an area roughly 40 miles northeast of our valley. However, even in our valley, many of these land ripples exist. Both in this picture and in our valley, these ripples are about 40 feet high.

Glacial Lake Missoula was the largest glacier lake known to have ever existed, and the final flood that resulted when the glacier dam burst was orders of magnitude greater than any river flood man has experienced in the last many centuries. It is hard to imagine the intensity of that flood, but it would be like suddenly releasing all of the water contained in Lake Erie and Lake Ontario today, all at once.

Be thankful that you weren’t around to see it.

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Written by Rex Trulove

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