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Glacial dams helped Tibet keep its cutting edge

October 9, 2008

October 8, 2008

PARIS (AFP) -- Boulders and soil left by retreating glaciers helped
preserve the mighty Tibetan plateau from being worn away by river
erosion, scientists said in a study released Wedneday.

The plateau resulted from the collision of the Indian and Asian
continental plates around 50 million years ago, creating some of the
world's greatest rivers and gorges and the most glaciated region
outside the north and south poles.

In a study published by the British-based science journal Nature, the
investigators looked at the Tsangpo River, the world's highest major
river, which starts at an elevation of 4,400 metres (13,500 metres)
before slicing through the Himalayas and emptying into the Bay of Bengal.

The Tsangpo cuts out vast quantities of soil and rock on its way to
the sea but in its upper reaches it has had strangely little erosive
effect on the edges of the Tibetan plateau.

The reason, say the team, is that over millions of years, glaciers
advanced and retreated over the region, depositing debris at the
mouth of many of the tributaries to the Tsangpo as they came and went.

These debris walls, called moraines, are acting as dams, preventing
the rushing 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," co-author David
Montgomery of the University of Washington said in a press release.

The findings are more than of academic interest, for they demonstrate
that some natural lakes in the southeastern margin of the Tibetan
plateau are in effect being held back by moraines.

Meltwater discharge from glaciers -- which is expected to accelerate
as a result of climate change -- could cause a swift and dangerous
buildup behind these natural dams.

"Some of these lakes may give rise to catastrophic outburst floods
with far-reaching consequences for both the local and downstream
population," lead author Oliver Korup, of the Swiss Federal Institute
for Snow and Avalanche Research, told AFP.

"This has been the case historically," he said.

"Quite a number of flooding disasters in Indian rivers have had their
cause in the sudden failure of nature dams in Tibetan headwaters. You
may imagine that this may generate quite some international friction."
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