New investigation indicates that among the biggest fresh-water floods within Earth’s history happened about 17,000 years ago and inundated a large area of Alaska that’s right now occupied in part through the city of Wasilla, widely recognized because associated with the 2008 presidential campaign.
The event had been one of at least 4 “megafloods” because Glacial Lake Atna breached ice public works and released water. The actual lake protected more than 3,500 square miles in the Copper River Basin northeast of Anchorage and Wasilla.
The megaflood which covered the Wasilla region introduced around 1,400 cubic kilometers, or 336 cubic miles, associated with water, enough to cover an area the size of Washington, D.C., to a depth of nearly 5 miles. Which water volume drained in the body of water within a week as well as, at this kind of great velocity, formed dunes more than 110 feet, along with at least the half-mile between crests. The actual dunes appear on topographical maps however nowadays are covered by roads, structures along with other improvement.
“Your mind doesn’t get around dunes of that size. Obviously the water had to be very deep to form them,” said Michael Wiedmer, an Anchorage native who is pursuing graduate studies in forest resources at the University of Washington.
Wiedmer is the lead author of a paper describing the Wasilla-area megaflood, published in the May edition of the journal Quaternary Research. Co-authors are David R. Montgomery and Alan Gillespie, UW professors of Earth and space sciences, and Harvey Greenberg, a computer specialist in that department.
This map shows the flood-formed dunes in the area of Wasilla, Alaska. Flood waters flowed from right to left across the image. The dunes reach more than 110 feet high and are spaced more than a half-mile apart.
By definition, a megaflood has a flow of at least 1 million cubic meters of water per second (a cubic meter is about 264 gallons). The largest known fresh-water ton, at regarding 17 million cubic meters per second, originated in Glacial Lake Missoula in Montana and had been among the series of major floods which created the actual Channeled Scablands of eastern Washington.
The megaflood from Glacial Lake Atna down what is now the Matanuska River to the Wasilla region might have had a flow of about 3 million cubic meters per second. Another suspected Atna megaflood along a different course to the Wasilla region, down the Susitna River, might have had a flow of about 11 million cubic meters per second. The researchers also found proof for two smaller Atna megafloods, down the Tok and Copper rivers.
Wiedmer, who retired from the Alaska Department of Fish and Game in 2006, began the research in 2005 when he discovered pygmy whitefish living in Lake George, a glacial lake 50 miles from Anchorage. The lake has essentially emptied numerous times in its history and was not thought to support much life. Study of physical characteristics show these fish tend to be more closely associated with pygmy whitefish within 3 other mountain lakes, all remnants of Lake Atna, than they are to any others of that species. Their existence in Lake George, some distance from the other lakes, is one piece of evidence for a megaflood from Lake Atna.
“Lake Atna linked up with four distinct drainages, and we think that helped it act like a pump for freshwater organisms,” he said.
The megaflood also could explain a few of the catastrophic damage which occurred in the magnitude 9.2 Great Alaskan Earthquake of 1964. Wiedmer noted that much of Anchorage is built on marine sediments, and one layer of those sediments liquefied and collapsed, allowing the layer above to slide toward the sea. As the upper coating shifted towards water, structures built along with it collapsed.
Although the marine sediments prolong about 200 feet deep, the failure only occurred within a narrow 3-foot layer. Researchers later on discovered that layer had been infused along with fresh water, which was unpredicted in sediments deposited below salt water. The old megaflood might account for the fresh drinking 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
