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Science

Researchers Find Alaskan Megaflood Happened 17,000 Years Too Early

New research from the University of Washington indicates that one of the planet’s largest fresh-water megafloods happened some 17,000 years ago and inundated a large area of Alaska.

[social_buttons]The comedic value of this story comes from the location of the megaflood, which flooded areas down what is now the Matanuska River in Alaska including Anchorage and Wasilla, home to the loveable Sarah Palin.

The discovery, made by Michael Wiedmer, an Achorage native who is pursuing graduate studies in forest resources at the University of Washington, is evidence of one of four megafloods that took place when Lake Atna breached her ice dams.

Wiedmer originally started his line of inquiry when he discovered pygmy whitefish living in Lake George, a glacial lake 50 miles from Anchorage, believed not to support much life as a result of the numerous times it has emptied over time. Examination of the fish showed that it had several physical traits similar to the pygmy whitefish in three other mountain lakes, all of which are remnants of Lake Atna.

Glacial Lake Atna was believed to cover 3,500 sqaure miles, or 4,800 square kilometres, in the Copper River Basin, northeast of Archorage and our very favourite Wasilla.

According to a paper that will be published in the May edition of the journal Quaternary Research, the megaflood that covered the Anchorage and Wasilla region released as much as 336 cubic miles, or 1,400 cubic kilomters of water. As a reference point, this much water is enough to cover an area the size of Washington, D.C. to a depth of nearly 5 miles (8 kilometres).

Co-authors of the paper are David R. Montgomery and Alan Gillespie, UW professors of Earth and space sciences, and Harvey Greenberg, a computer specialist in that department.

The water drained from Glacial Lake Atna in about a week at about 4 million cubic meters per second forming dunes that stood higher than 110 feet, with at least a half-mile between crests. “Your mind doesn’t get around dunes of that size. Obviously the water had to be very deep to form them,” said Wiedmer.

By definition, a megaflood has a flow of at least 1 million cubic meters per second. The largest of these floods occurred in Glacial Lake Missoula in Montana and is one of a series of cataclysm floods which formed the Channeled Scablands of eastern Washington. Another suspected Lake Atna megaflood which took a different course along the Susitna River might have had a flow of about 11 million cubic meters per second. Two more smaller Atna megafloods are suspected to have journeyed down the Tok and Copper rivers.

There is more evidence for the Lake Atna megafloods as well. The Great Alaskan Earthquake of 1964 which recorded a magnitude 9.2 on the Richter scale saw a layer of marine sediments under Anchorage liquefy and collapse, allowing the layer above to slide towards the sea causing structures built on that layer to collapse.

However, despite the fact that the marine sediments extend about 200 feet deep, the failure only occurred within a narrow 3-foot layer of sediment, which scientists later discovered had been infused with fresh water, an unexpected phenomenon in sediments deposited under salt water. Wiedmer and his colleagues believe the ancient megaflood could account for the salt water.

“We suspect that this is evidence of the flood that came down the Matanuska,” Wiedmer said. “The location is right at the mouth of where the flood came down, and the time appears to be right.”

Source: University of Washington
Image Source: Susan Winchell-Sweeney; Laurie Rush, PhD, Editor




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