Science Gazette

Ancient megafloods were regulated by the tilting of the Earth’s crust

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When the last ice age ended, a series of cataclysmic floods known as the Missoula megafloods swept across eastern Washington, cutting long, deep channels and towering cliffs through a region now known as the Channeled Scablands. They were among the world’s greatest known floods, and geologists working to replicate them have discovered a key component controlling their flow.

The shifting weight of the ice sheets would have forced the whole terrain to tilt, affecting the route of the megafloods, according to a study published February 14 in Proceedings of the National Academy of Sciences.

“People have been looking at high water marks and attempting to recreate the extent of these floods,” said lead author Tamara Pico, an assistant professor of Earth and planetary sciences at UC Santa Cruz. “But all of the estimations are based on looking at the present-day terrain.” “The deformation of Earth’s crust by the weight of the ice sheets would have caused the ice age topography to be different across wide scales,” says the report.

Huge ice sheets blanketed most of North America toward the end of the previous ice age. After nearly 20,000 years, they started to melt, and the Missoula megafloods occurred between 18,000 and 15,500 years ago. Pico’s team looked at how the shifting weight of the ice sheets changed the landscape of eastern Washington throughout this time, affecting how much water flowed into various rivers during the floods.

When a lobe of the Cordilleran ice sheet blocked the Clark Fork valley in the Idaho panhandle and melt water pooled up behind the barrier, Glacial Lake Missoula developed in western Montana. The water eventually rose to the point that the ice dam started to float, causing a glacial outburst flood. The ice dam resettled and the lake refilled once enough water had been released. Over the course of thousands of years, this procedure is estimated to have occurred hundreds of times.

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The Columbia River was blocked by another ice lobe downstream of glacial Lake Missoula, becoming glacial Lake Columbia. The water spilled over to the south into the eastern Washington plateau as Lake Missoula’s outburst floods surged into Lake Columbia, destroying the terrain and forming the Channeled Scablands.

The deformation of the Earth’s crust in reaction to the increasing and shrinking of ice sheets would have altered the terrain by hundreds of meters throughout this time, according to Pico. Her team used flood models to study how the tilting of the topography affected megaflood routing and erosional power in various channels.

“We utilized flood models to anticipate water velocity and erosional force in each channel, then compared it to what would be required to erode basalt, the kind of rock found on that region,” Pico said.

They concentrated on the Cheney-Palouse and Telford-Crab Creek areas, two significant channel systems. Early floods would have degraded both tracts, but subsequent floods would have focused the flow in the Telford-Crab Creek system, according to their findings.

“As the topography tilted, it influenced both where the water spilled out of Lake Columbia and how water flowed through the channels, but the spillover into those two tracts was the most critical consequence,” Pico said. “What’s fascinating is that topography isn’t static, so we can’t just recreate the past using today’s topography.”

According to her, the results provide a fresh viewpoint on this interesting setting. The huge floods imprinted numerous extraordinary characteristics onto the ground, including steep canyons hundreds of feet deep, dry falls, gigantic potholes, and ripple imprints.

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“It’s astonishing to think of the volume of the floods that were required to cut those canyons, which are now dry,” Pico added. “There are also massive dry waterfalls—an it’s impressive scene.”

She also said that catastrophic floods are mentioned in the oral histories of Native American tribes in this area. Pico said, “Scientists were not the first to look at this.” “Perhaps people were there to observe the floods.”

Scott David of Utah State University, Isaac Larsen and Karin Lehnigk of the University of Massachusetts, Amherst, Alan Mix of Oregon State University, and Michael Lamb of the California Institute of Technology are among the coauthors. The National Science Foundation provided funding for this research.

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