#33 - Shorelines of Glacial Lake Missoula

I took this photo from Mt. Sentinel (the one with the "M"), looking across the Clark Fork River at Mt. Jumbo (the one with the "L" for Loyola Sacred Heart High School). The horizontal lines are shorelines on ancient Glacial Lake Missoula.

Here's what happened . . .
As the climate began to get colder, glacial ice grew southward from the Arctic, covering Canada with a continental glacier that was over two miles thick in places. At its maximum (about 18,000 years ago) the ice sheet advanced as far south as New York City, Ohio, and central Montana. In central Montana the present-day Missouri River marks the southern edge of the glacier. As the ice extended into western Montana and Idaho, it blocked the flow of the Clark Fork River. The Clark Fork starts near Butte, flows north through Missoula, and then into Idaho. Eventually its waters empty into the Columbia River, which flows into the Pacific Ocean near Portland. As the ice blocked the flow of the Clark Fork near Pend Oreille Lake, water began to build up behind (south of) the ice dam. This formed a huge lake that geologists have named "Glacial Lake Missoula."

Inland sea . . .
When the Lake Missoula was at its highest, the water was about 2,000 feet deep and contained about as much water as Lake Erie. The lake extended as far south as Drummond in the Clark Fork Valley and Darby in the Bitterroot Valley. But, once the water filled in the area behind the ice dam, the lake didn't last for long. Since ice floats, it doesn't make for a very durable dam. Consequently, it was only a matter of time before the lake dislodged its ice dam. With the dam displaced, the 480 cubic miles of water impounded behind it would have been unleashed in a cataclysmic flash flood of incredible proportions. Starting at Pend Orielle Lake the water would have thundered through present-day Spokane and continued across eastern Washington to the Columbia River, scouring the land as it swept through.

Over and over . . .
Geologists think that this happened many times. Once the front of the glacier was swept away by the water it had impounded, the lobe of ice grew back into the area and re-dammed the river. Geologists believe that at Lake Missoula formed and flooded at least 41 times between 15,500 and 13,200 years ago.

Below: Glacial Lake Missoula wasn't the only glacial lake. Several others existed along the southern edge of the ice.

Modified from Fullerton and other, 2012; Hyndman and Thomas, 2020

Term: outburst flood