Glacial dams helped prevent erosion of Tibetan plateau

October 9th, 2008 - 1:44 pm ICT by ANI  

Washington, Oct 9 (ANI): A new research has suggested that the Tibetan plateau might have been preserved for thousands of years by glacial dams that helped prevent erosion.
The Tsangpo River is the highest major river in the world, starting at 14,500 feet elevation and plunging to the Bay of Bengal, scouring huge amounts of rock and soil along the way.
Yet, in its upper reaches, the powerful Tsangpo seems to have had little effect on the elevation of the Tibetan Plateau.
New research suggests that the plateau edge might have been preserved for thousands of years by ice during glacial advances and by glacial debris deposited at the mouth of many Tsangpo tributaries during warmer times when glaciers retreated.
Those debris walls, or moraines, acted as dams that prevented the rapidly traveling water in the main Tsangpo gorge from carving upstream into the plateau.
“At the edge of the plateau, the river’’s erosion has been defeated because the dams have flattened the river’’s slope and reduced its ability to cut into the surrounding terrain, making it more like a lake,” said David Montgomery, a University of Washington geomorphologist.
The researchers focused on the three primary rivers of the Tsangpo system, the Yarlung Tsangpo and its two major tributaries, the Yigong Tsangpo and the Parlung Tsangpo.
The scientists mapped geologic evidence of more than 300 natural dams, including 260 moraines, that have formed repeatedly at the mouths of tributaries in the last 10,000 years to block water flow on the three main streams.
The first evidence of the dams was found at the edge of the Tibetan Plateau, and additional evidence continued to be found upstream, according to Montgomery.
The dams essentially formed giant lakes along the river and prevented the water from carving into bedrock.
“The glaciers seem to have helped preserve the edge of the plateau by keeping the river from ripping into it,” said Montgomery. “This isn”t the explanation for why the rest of the plateau is so well preserved, but it might work for this area where the Tsangpo crosses the edge of the plateau,” he added.
According to the researchers, the glacial dams on tributaries right to the edge of the plateau prevented erosion.
“It’’s a transition from where the river is doing all the erosion at lower elevations to where the glaciers are doing all the erosion at high elevations, and the glaciers are limited on how deeply they erode,” Montgomery said.
“They shave off the top but they don”t erode farther down, and the rivers can”t erode back past the glaciers,” he added. (ANI)

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