Wednesday, 30 May 2012

The Formation and Fate of World's Largest Canyon

The steep topography along the Yarlung-Tsangpo River ( (Tibetan: ཡར་ཀླུངས་གཙང་པོ་, Wylie: yar kLungs gTsang po) is created due to the interplay between the forces of tectonics and powerful river erosion, which in turn leads to large landslides.

This is the conclusion drawn by Isaac J. Larsen and David R. Montgomery from the University of Washington (presented online May 27, 2012 in Nature Geoscience) after quantifying landslide erosion rates in the eastern Himalaya. 

They closely observed an area of the 150-mile Tsangpo Gorge in southeastern Tibet, where the Tsangpo plunges more than 6,500 feet (1.25 miles), before entering India to form Brahmaputra River and flow into Bay of Bengal through the Ganges River delta.

Map of the Yarlung Tsangpo River watershed which drains the north slope of the Himalayas. The Yarlung Tsangpo is sometimes considered the upper section of the Brahmaputra River in northeastern India, which flows to the Bay of Bengal and the Indian Ocean. Credit: Wikipedia

The Great Tsangpo Gorge is believed to be the largest canyon in the world with a depth  of 5,382 m (17,657 ft) and a total length of 496.3 km. The Gorge falls in the Yarlung-Tsanpo Suture zone which experiences strike-slip motion (when two plates slide past each other after collision) of different continental fragements within the Tibetan Plateau. Gyatsa Canyon in the middle reaches of the river experieces high degree of tectonic activity.

The two researchers found that the rapidly flowing water of Tsangpo erode soil from the base of slopes leaving exposed bedrock and an increased slope angle (>30 degrees) that triggers landslides to stabilize the slopes . From 1974 through 2007, erosion rates reached more than a  half-inch per year along some 6-mile stretches of the river within the gorge, and throughout that active landslide region erosion ranged from 0.15 to 0.8 inch per year.

The Landsat satellite image at left shows a huge lake on the Tsangpo River behind a dam created by a landslide (in red, lower right of the lake) in early 2000. The image at right shows the river following a catastrophic breach of the dam in June 2000. Credit: U.S. Geological Survey/NASA

Images (above) shows that a huge landslide in early 2000 created a gigantic dam on a stretch of the Tsangpo. The dam failed catastrophically in June of that year, and the ensuing flood caused a number of fatalities and much property damage downstream. This illustrates that the processes on steep mountain terrain occur on a faster timescale in the Tsangpo Gorge than in the other steep mountain regions of the world.

In this context, it is noteworthy that the Chinese government has started to build a dam with 510 MW of installed capacity, on Yarlung Tsangpo River near rZam  (Tib:ཛམ་; Ch: Zangmu)་with series of at least 5 more dams in the pipeline. These dams are being built in the region with high tectonic activity and seismic vulnerability caused by north-south trending normal faults and east-west trending suture zones. The tectonic activities and landslides events in the region could be further accelerated by the construction of these dams which leads to structural imbalance, lubrication of faults and back-water incision. Any damage to and by the dam could have catastrophic results in the lower riparian (downstream) region.

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