At the end of the Earth's last Ice Age, as ice sheets melted, a number of succinct cataclysmic floods known as the Missoula megafloods scrubbed the landscape of what is now eastern Washington. The floods carved long deep channels and towering cliffs through the Channeled Scablands area. The floods were among the largest floods in Earth's known history. Geologists today struggle to reconstruct the devastation and direction of the floods. However, in a recent study, a team of researchers has successfully identified a vital factor that governed the flow and direction of these ancient megafloods.
Ebb and Flow of the Missoula Megafloods
GLACIER NATIONAL PARK, MONTANA - SEPTEMBER 17: With Lake McDonald 4,300 feet below, trees burned by the 2017 Sprague Creek Fire stand along the steep trail to the Mt. Brown Lookout Station September 17, 2019, in Glacier National Park, Montana. A U.S. Park Service revegetation crew planted 585 two-year-old whitebark pine seedlings among the skeletal remains of this forest because the tree grows more successfully in the ground that was recently burned. With annual average temperatures in Montana rising almost three degrees Fahrenheit since 1950, high elevation tree species like the whitebark pine that were not previously threatened are now facing an increase in blister rust infections, mountain pine beetle infestations, and wildfire. A slow-growing species that live at elevations above 6,000 feet, the whitebark is an essential food source for many birds and small mammals.
In a recent study published in the journal Proceedings of the National Academy of Sciences, titled "Glacial isostatic adjustment directed incision of the Channeled Scabland by Ice Age megafloods," researchers demonstrated how the changing weight of the ice sheets during its melting would have caused the landscape to tile, thereby changing the course of the Missoula megafloods.
Tamara Pico, lead author and an assistant professor of Earth and Planetary Sciences at the University of California Santa Cruz explains that researchers have long been looking at high water marks in order to reconstruct the size of the Missoula floods. However, all the estimates have been based on the current topography of the landscape.
She adds that the paper showcases how the ice age topography would have been widely different over broad scales compared to present day due to the deformation of the planet's crust by the weight of the said ice sheets.
In the height of the planet's last Ice Age, vast ice sheets engulfed a majority of North America. These ice sheets started melting after roughly 20,000 years in the past, while the Missoula megafloods occurred from 18,000 to 15,500 years ago. Pico and her colleagues analyzed how the changes in weight of the ice sheets in the said period would have tilted the landscape of eastern Washington, and how the said changes would have influenced the ebb and flow of the floods.
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How Changes in the Weight of Ice Sheets Tilted the Earth's Crust Changing the Direction of the Missoula Floods
After a lobe of the Cordilleran ice sheets dammed the Clark Fork Valley in Idaho, where melted water built up, the Glacial Lake Missoula was formed in western Montana. Eventually, the built-up water got so deep that the ice began floating, resulting in a flood of glacial. The ice dam settled when enough water had been freed, and the lake was then refilled. This process is believed to have repeated countless times over several thousand years, reports PhysOrg.
Downstream from the Glacial Lake Missoula, the Columbia River was then dammed by another lobe of ice which formed glacial Lake Columbia. When the outbursts of floods from Lake Missoula poured into Lake Columbia, the water spilled to the south and eastern Washington plateau, resulting in the erosion of the landscape and creating the Channeled Scablands.
For thousands of years, the deformation of the planet's crust in response to the changing weight of ice sheets would have also changed the elevation of the landscape by hundreds of meters, according to Pico. Together with her researchers, they incorporated the said changes into flood models to analyze how the titling of the landscape changed the routing of the Missoula megaflood and their erosion power in various channels.
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