37.5: Glacial Lake Missoula

Above the town of Missoula, Montana are a series of subtle notches in the hillside. Like the giant ripples we mentioned earlier, they are hard to see if you’re standing on them. They show up most photogenically when lightly dusted with snow, and viewed from high above.

Figure \(\PageIndex{1}\): Strandlines from Glacial Lake Missoula wrap around Mount Jumbo above Hellgate Canyon. (Image modified from a still from a video produced by the Montana Natural History Association.)

Each of these notches is perfectly horizontal, wrapping around the landscape without deviating in elevation even by an inch. Thomas Crowder Chamberlin noted these in 1886, correctly interpreting them as ancient lakeshores by comparing them to what he had read of the so-called “Parallel Roads” of Glen Roy, Scotland. The idea is that waves on the shore of a now-vanished lake must have lapped against the hillside with sufficient vigor to etch out a notch. The valley was apparently filled by lakes of many different levels in the past. These lakeshores are called strandlines.

Figure \(\PageIndex{2}\): Map of Glacial Lake Missoula at its largest extent. Red lines are major roads. (Modified by CB from an original by the Montana Natural History Association.)

The Missoula strandlines imply an ancient lake once filled the valley where today the University of Montana is situated adjacent to the Clark Fork River. What formed that ancient lake? J.T. Pardee thought he knew. Pardee was a native Montanan who pursued a career doing geology for the United States Geological Survey. He traced out the strandlines across the landscape, heading north and downstream along the Clark Fork River. His mapping led him to Sandpoint, Idaho (on the Idaho/Montana border), where there was no evidence of a landslide dam, nor a lava dam, nor any other kind of dam that was still there. What impounded the Clark Fork to make the lake, then? If the Parallel Roads of Glen Roy were to act as any guide, perhaps it was glacial ice that dammed the river.

Pardee postulated that the Purcell Lobe of the great Laurentide ice sheet nudged south into the valley and made an ice dam 23 miles (37 km) wide, spanning the relevant drainage, and rising to half a mile (0.8 km) high above the valley floor. The impounded waters of the Clark Fork River pooled to make Glacial Lake Missoula. This huge glacial lake extended up to 200 miles upstream from the ice dam.

Figure \(\PageIndex{3}\): Varves in laminated mudstones deposited on the floor of Glacial lake Missoula. (Callan Bentley photo.)

In places, the lake may have been as deep as 200o feet. At the bottom of this vast volume of water, depositional conditions appeared calm. In many places, laminated strata record lake-bottom sedimentation that alternated rhythmically between silt and clay. These regular oscillations are interpreted as varves.

Why did the dam eventually break? The answer lies with the fact that ice is less dense than liquid water. So when the voluminous waters of the lake got deep enough, this giant ancient lake simply floated its icy dam away. Released by the now-absent dam, where did the waters of Glacial Lake Missoula go? In a word, downhill.

The path downhill — to the Pacific Ocean, via the Columbia River — ultimately led across eastern Washington. Released in an instant, the violent draining out of the lake unleashed 500 cubic miles of water across the landscape. With a discharge estimated at 60 times the size of the Amazon River, the flood waters gouged in and spread out, and reconvened, and pushed boulders, deposited giant ripples, tumbled over waterfalls, and drilled out massive potholes.

And then the ice flowed back in, rolling over the same ground, backing up the same river again, regenerating a new iteration of Glacial Lake Missoula, and etching a new strandline into the surrounding valley walls.

This 2015 video, by Nick Zentner and Tom Foster, shows many of the key details: