July 23, 2013
Geochemical Evidence Suggest Ancient Megafloods Eroded Tsangpo Gorge
April Flowers for redOrbit.com - Your Universe Online
Dropping rapidly through the Himalaya Mountains on its way to the Bay of Bengal, the Yarlung-Tsangpo River in southern Asia loses about 7,000 feet of elevation through the precipitously steep Tsangpo Gorge.
"You would expect that if a three-day long flood occurred, there would be some pretty significant impacts downstream," said Karl Lang, a UW doctoral candidate in Earth and space sciences. The findings of this study were published in a recent issue of the journal Geology.
The water that moved through the bedrock gorge moved rapidly, carving away the base of slopes so steep, they were already near the failure threshold. The riverbed running through Tsangpo Gorge is essentially bedrock, with a slope so steep and narrow that the deep floodwaters could build enormous speed and erosive power.
Areas higher on the bedrock walls tumbled into the channel as the base of the steep slopes eroded. This freed microscopic grains of zircon to be carried out of the gorge by the fast-moving water and deposited downstream.
Zircon grains are uranium-bearing and carry the geochemical signature of the place where they originated. Grains downstream, therefore, can be traced back to the rocks from which they eroded. The researchers found normal annual river flow carries approximately 40 percent of the grains from Tsangpo Gorge downstream. The grains from the gorge found in prehistoric megaflood deposits, however, make up as much as 80 percent of the total.
The highest major river in the world, the Yarlung-Tsangpo begins on the Tibetan Plateau at about 14,500 feet, or more than 2.5 miles, above sea level before traveling over 1,700 miles. It crosses the plateau and plunges through the Himalayas before reaching India's Assam Valley, where it becomes the Brahmaputra River and continues its course to the Ganges River delta and the Bay of Bengal.
The river makes a sharp bend around Namche Barwa, a 25,500-foot peak that is the eastern anchor of the Himalayas, at the head of the Tsangpo Gorge. At various times during the last 2.5 million years, evidence indicates giant lakes were impounded behind glacial dams farther inland from Namche Barwa.
The research team matched zircon grains in the megaflood deposits far downstream with grains known to come only from Namche Barwa. Those specific zircon grains showed up in the flood deposits at a much greater proportion than they would in sediments from normal river flows. Finding them in deposits so far downstream is strong evidence of the prehistoric megafloods and their role in forming the gorge.
A huge landslide in early 2000 created a dam on the Yiggong River, the team noted. The Yiggong is a tributary of the main river, just upstream from the Gorge. That dam suffered a catastrophic failure in June 2000, which triggered a flood that caused numerous fatalities and much property damage downstream.
This dam failure provided a much smaller, but vivid, illustration of what likely occurred when large ice dams failed millions of years ago. It also demonstrates the potential hazard if humans decide to build dams in that area for hydroelectric generation.
"We are interested in it scientifically, but there is certainly a societal element to it," Lang said. "This takes us a step beyond speculating what those ancient floods did. There is circumstantial evidence that, yes, they did do a lot of damage."
The team notes the process present in the Tsangpo Gorge is similar to that of Lake Missoula in Western Montana 12,000 to 15,000 years ago. Lake Missoula was more than 10,000 feet lower in elevation than the waters associated with the Tsangpo Gorge. However, its water discharge was ten times greater. The ice dam at Lake Missoula failed numerous times, evidence suggests, unleashing a flood equal to half the volume of Lake Michigan across eastern Washington, where it carved the Channeled Scablands before continuing down the Columbia River basin.
"This is a geomorphic process that we know shapes the landscape, and we can look to eastern Washington to see that," Lang said.