How a vanished Ice Age lake formed the previous and current of Missoula, Montana

Image of a grassy hillside with a series of natural steps cut into it.
Enlarge / Previous shorelines left deposits which might be nonetheless seen on the hills close to the Missoula Valley.

Richard Forbes

Had the town of Missoula, Montana, existed 1000’s of years in the past, it will have been below water.

Over the last Ice Age, a sheet of ice 20 miles extensive bought caught within the Idaho panhandle and blocked the Clark Fork River, creating glacial Lake Missoula. At its highest, the water degree reached 4,250 ft above sea degree—over 1,000 ft above the current metropolis’s altitude. The ice sheet in the end gave option to the strain of the water, and glacial Lake Missoula drained catastrophically.

It’s estimated that the most important flood discharge reached 386 million cubic feet per second. At that fee, it took the lake only some days to empty, with its waters ultimately reaching the Pacific Ocean.

The scariest factor is just not the size of this occasion—it is that floods of this dimension occurred a number of instances. Hundreds of years after that first flood, scientists lastly pieced collectively whether or not it was a one-time deal by trying to the dust for solutions.

Indicators of the previous

Within the fall of 1969, Wealthy Chambers drove southwest of Missoula alongside I-90 along with his undergraduate adviser. They pulled over to the aspect of the street to take a look at a wall that ran 80 ft into the air. It was zebra-striped, with layers of darkish and light-weight sediments working horizontally up the slope.

Missoula is the second-biggest metropolis in Montana, with a inhabitants of about 75,000, and it sits in a mountain valley. The College of Montana is understood extra for its forestry and regulation faculties and fewer for the enormous boulders sitting round campus or the strains on two mountains—seen from about anyplace on campus—which might be remnants of the lake that when drowned the valley.

Chambers devoted his undergraduate and grasp’s work to glacial Lake Missoula, which fashioned behind the Cordilleran Ice Sheet between 14,000 and 21,000 years in the past. The glacial lake would cowl virtually 3,000 square miles and maintain as a lot water as Lake Ontario and Lake Erie combined.

Chambers’ adviser was David Alt, a well-renowned historian of glacial Lake Missoula. One thing Alt wasn’t as accustomed to, although, was the sediments left behind after the flood drained the lake—the form of sediments that he and Chambers discovered themselves taking a look at off I-90.

“These are Lake Missoula sediments,” Alt mentioned to Chambers as they stared on the zebra wall. “And no person’s taking a look at them intimately.”

If there was to be something new uncovered concerning the lake’s historical past, it will come from sediments like these. And there was some urgency in uncovering it—within the Seventies, there was a giant back-and-forth brewing within the scientific neighborhood relating to what number of instances the lake could have drained and refilled.

Land of many lakes

Chambers began classifying Lake Missoula’s sediments and observed two scales of zebra striping. On the massive scale, he discovered about 40 alternating mild and darkish soil sequences as much as a number of meters thick. These cycles, known as rhythmites in geology-speak, are deposits the place the sunshine layers are made up of wonderful sand and silt deposited by rivers within the early levels of a lake’s filling, whereas the darkish layers are made up of silt and clay that collect on the underside of crammed lakes.

Chambers then observed that the darkish layers had their very own zebra stripes. The stripes-within-stripes at this scale are known as varves, they usually doubtless represented annual layers of sediment stacked on prime of one another. The varves advised geologists concerning the period of time it took for the lake to fill. Assuming the varves represented annual stackings, Chambers mentioned it took solely a median of about 50 years for the lake to refill. Although the lake’s depth decreased with every filling, that’s nonetheless a wild quantity of water. At its peak, it was greater than 500 cubic miles of water, about half the amount held in Lake Michigan.

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

Zebra stripes in a deposit that when fashioned on the lake backside.

Wealthy Chambers

Little by little, Chambers and Alt pieced collectively an image of the realm’s historical past. In a paper that Chambers and Alt co-authored, they recommended a number of a long time might need handed between every lake draining and filling; in a special paper, Chambers concluded there was no proof that the glacial lake had drained utterly every time. He later argued that the final a number of lake drainages have been much less intense, which saved the floods from washing away deposits just like the one seen on the roadcut on I-90.

That piece of land off the freeway comprises at the least 800 years of historical past—and probably extra. It’s unclear how a lot sediment could have been faraway from the report by subsequent floods. Every draining of glacial Lake Missoula may have taken with it proof of earlier floods.