How a Vanished Ice Age Lake Shaped Missoula, Montana’s Past and Present

Enlarge / Past banks have left deposits that are still visible on the hills near the Missoula Valley.

Richard Forbes

If the city of Missoula, Montana had existed thousands of years ago, it would have been under water.

During the last ice age, a 20-mile-wide sheet of ice wedged into the Idaho Panhandle and blocked the Clark Fork River, creating Glacial Lake Missoula. At its highest, the water level was 4,250 feet above sea level, more than 1,000 feet above the city’s current elevation. The ice sheet eventually gave way to water pressure, and Glacial Lake Missoula drained catastrophically.

The greatest flood flow is estimated to have reached 386 million cubic feet per second. At this rate, it only took a few days for the lake to drain, with its waters eventually reaching the Pacific Ocean.

What is most frightening is not the scale of this event, it is that floods of this magnitude have occurred several times. Thousands of years after that first flood, scientists finally figured out if it was a one-time deal as they searched for answers.

Signs of the past

In the fall of 1969, Rich Chambers drove southwest from Missoula along I-90 with his undergraduate adviser. They stopped at the side of the road to look at a wall that stood 80 feet in the air. It was striped, with layers of dark and light sediment running horizontally down the slope.

Missoula is the second largest city in Montana, with a population of approximately 75,000, and it sits in a mountain valley. The University of Montana is better known for its forestry and law faculties than for the giant boulders that surround the campus or the lines on two mountains – visible from almost anywhere on campus – which are remnants of the lake that once drowned the valley.

Chambers devoted his undergraduate and master’s work to Glacial Lake Missoula, which formed behind the Cordilleran Ice Sheet between 14,000 and 21,000 years ago. The glacial lake would cover nearly 3,000 square miles and hold as much water as Lake Ontario and Lake Erie combined.

Chambers’ advisor was David Alt, a renowned historian of Glacial Lake Missoula. Something Alt wasn’t as familiar with, however, was the sediment left behind after the flood drained the lake — the kind of sediment he and Chambers found themselves staring at off I-90.

“It’s sediment from Lake Missoula,” Alt told Chambers as they looked at the wall of zebras. “And no one looks at them in detail.”

If there were to be anything new discovered about the history of the lake, it would come from sediments like these. And there was an urgent need to find out – in the 1970s there was a big back and forth in the scientific community regarding how many times the lake could have drained and refilled.

Land of many lakes

Chambers began classifying the sediments of Lake Missoula and noticed two scales of zebra stripes. On a large scale, he found about 40 sequences of alternating light and dark soils up to several meters thick. These cycles, called rhythmites in geology, are deposits where the light layers consist of fine sand and silt deposited by rivers at the start of a lake’s filling, while the dark layers consist of silt and clay which accumulate on the bottom of filled lakes.

Chambers then noticed that the dark layers had their own zebra stripes. Stripes within stripes at this scale are called varves, and they probably represented annual layers of sediment piled on top of each other. The varves explained to geologists how long it took for the lake to fill. Assuming the varves represented annual stacking, Chambers said it only took about 50 years on average for the lake to fill. Even though the depth of the lake has decreased with each filling, it’s still a wild amount of water. At its peak, it was over 500 cubic miles of water, about half the amount in Lake Michigan.

Zebra stripes in a deposit that once formed at the bottom of the lake.

Zebra stripes in a deposit that once formed at the bottom of the lake.

Rich rooms

Gradually, Chambers and Alt pieced together a picture of the region’s history. In a paper co-authored by Chambers and Alt, they suggested that several decades could have passed between the draining and refilling of each lake; in a different paper, Chambers concluded that there was no evidence that the glacial lake emptied completely each time. He later argued that the last drainages from the lake were less intense, preventing flooding from washing away deposits like the one seen at the roadcut on I-90.

This piece of land near the highway contains at least 800 years of history, and possibly more. It is not known how much sediment may have been removed from the record by subsequent flooding. Each drainage of Glacial Lake Missoula could have carried with it evidence of earlier flooding.

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