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

Enlarge / Past shorelines 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 stuck in 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 cap 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, and its waters eventually reached the Pacific Ocean.

The most frightening thing 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 advisor. 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 is located in a mountain valley. The University of Montana is best known for its schools of forestry and law and less 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 is said to 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. /p>

"It's sediment from Lake Missoula," Alt told Chambers as they stared at the wall of zebras. "And no one looks at them in detail."

If there were to be anything new to discover about the history of the lake, it would come from sediments like these. And it was urgent to find out - in the 1970s there was a great back and forth in the scientific community regarding how many times the lake had been able to drain and refill.

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 800 km3 of water, about half the amount in Lake Michigan.