Purcell Lobe
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The Purcell Lobe

Catastrophic Flooding in the Pacific Northwest

Published in the EarthSphere Blog. Feature Image: The Columbia River Gorge Sans Missoula Flood Water (©2022 Archean Enterprises, LLC; ArcheanArt)

A text recently popped up from my long-time friend, fellow geologist, and nature photographer CF Lovelace. He pointed me to a recent article by Rebecca Latson — Exploring Along The Ice Age Floods National Geologic Trail — Part 1. It was a great article with wonderful photographs detailing fascinating geomorphic features created by the Missoula floods. These floods started in Montana and ravaged large portions of the Pacific Northwest near the end of the last ice age. I would highly recommend the article.

But one aspect of these historic floods the article didn’t delve into was their geological/hydrological origins. The story started in Western Montana about 15,000 years ago. Earth was in the throes of its most recent ice age, and ice dominated the Northern Hemisphere. Large glaciers pushed southward from the Arctic, traversing Canada and pushing massive lobes of glacial ice into the northern regions of the US.

One particular glacial extrusion, the Purcell Lobe, occupied northern Idaho and the northwestern edge of Montana. The Purcell Lobe was notable because it blocked off the Clark Fork River, creating a massive ice dam over 2000 feet tall. The dam caused water to back up in western Montana, forming Lake Missoula, a glacial lake covering about 7,770 square kilometers.

Stopping Water with Water

It doesn’t take much imagination to understand the pitfalls of using water to create a dam. This trick only works in extremely cold conditions. Lake Missoula was always destined to be a geologically ephemeral feature whose nemeses were heat and pressure.

The heat part is self-evident since the ice age was ending and Earth was on a warming trend. The pressure problem has to do with the impressive height of the dam. Freshwater, having a specific gravity of 1 and a density of 1 g/cc, exerts a pressure gradient of 0.433 psi/foot, translating to pressures of over 850 psi at the base of the Purcell Lobe ice dam.

Inevitably, as temperatures increased, the ice weakened. At some point, the dam could no longer withstand the pressure from the Lake Missoula water, and it burst, creating the first of the Missoula Floods. Approximately 2,100 cubic kilometers of water, half the volume of Lake Michigan, came pouting out of Montana, hell-bent on reaching the Pacific Ocean as quickly as possible.

The flood water ripped across eastern Washington, crossed the border to Oregon at Wallula Gap, and proceeded due west through the Columbia River Gorge. It was truly the flood from hell, flowing with the force of 60 Amazon Rivers. But it created the spectacular scenery photographed in Rebecca Latson’s article.

Ice ages don’t end overnight, and after the first flood, the dam reformed, and the whole process repeated itself many times before the Purcell Lobe finally melted away.

Lake Allison

Flood waters raging through the Columbia River Gorge are estimated to have been up to 1,000 feet deep during the largest floods. They ripped away the mountainsides creating the beautiful scenery we enjoy today during a drive through the Columbia River Gorge National Scenic Area.

When the water exited the Gorge, it charged westward towards Portland, where it encountered a 90-degree bend in the Columbia River, forcing the torrents of water north for 40 miles and then west again before emptying into the Pacific.

This twisting and turning, along with the narrows at Kalama Washington, created a hydrologic traffic jam, causing the flood waters to back up into the Willamette Valley. The resulting lake, Lake Allison, stretched about 100 miles to the south and reached depths of over 400 feet.

I live at the northern end of the Willamette Valley, and each day as I descend into the valley towards I-205, I try to estimate at what point my car would have plunged below the water of Lake Allison.

The end of the ice age was a dramatic time for the Pacific Northwest. We owe much of the beauty we encounter today to the catastrophic Missoula Floods.


William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.