Catastrophic erosion in Tibet

The world’s most awesome natural spectacle is probably the Brahmaputra River in full spate.  Unlike most large rivers, it is constrained for most of its course within a deep, narrow gorge that has to take the snow melt from a huge catchment on the northern flank of the High Himalaya, brought partly by the Tibetan Tsangpo River.  Each spate hurtles onto the plains of  Bangladesh, loaded with debris, at a rate of around 70 thousand cubic metres per second.  Although that is but a third of the flood discharge of the Amazon, for much of the  Brahmaputra’s course it must pass through a gorge only a few hundred metres wide in places.  This gives not inconsiderable erosive power, indeed probably the highest anywhere.  Not surprisingly, little is known about the Tsangpo-Brahmaputra valley, because of its inhospitable character.  With the recent release of ~90m resolution elevation data from the Shuttle Radar Topography Mission, it is now possible to analyse the whole catchment’s morphology in detail, without needing to follow the individual rivers.  Parts of the lower Tsangpo have remarkably high gradients, including a 100 km stretch with a fall of more than 2 km, through a gorge with almost 7 km of relief on either flank that cuts N-S across the axis of the Eastern Syntaxis of the High Himalaya. The gorge lies downstream of a west to east stretch with lower gradients, falling around 1 km in 300 km, which suggests some dramatic incision begins at the junction of the two sections.  US and Chinese geomorphologists visited the area and discovered that high on the flanks of the upper Tsangpo are terraces of lacustrine sediments, at about 3100 and 3500 m (200 and 600 m higher than the river) (Montgomery, D.R. et al. 2004.  Evidence for Holocene megafloods down the Tsangpo River gorge, southeastern Tibet.  Quaternary Research, 9 September 2004 issue).  Charcoal in the sediments gives radiocarbon ages between 1200 to 1600 BP and 8800 to 9800 BP for the lower and higher terrace levels, so the lakes formed during the Holocene.  The terraces stop at a zone of thick glacial moraine, cut by the Tsangpo, which suggests that both formed in lakes behind two ice dams.  Using SRTM data allows the volume of water ponded in both ice-dammed lakes to be estimated.  The older and higher level indicates about 830 km3, and the lower some 80 km3.  Breaching of the dams would have caused the largest recorded erosive events in recent Earth history, and explains the gorge below.  Each flood discharge would have been between 1 and 5 million cubic metres per second, equivalent 3 to 15 times the maximum flood discharge of the Amazon.

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