Sixty miles east of Mount Kailash, a stream trickles down from the Chemay-ungdung mountain range. Known as the Tamchok or &lsquohorse river&rsquo in Western Tibet, this is the source of the mighty Brahmaputra. Changing names with an easy whimsy, the river becomes the Tsangpo as it surges east for more than a thousand miles across Tibet&rsquos vast plateau, till it hits the snow-capped Namche Barwar (7,765m). The mountain blocks its progress due east, making the river loop around its base and giving rise to the &lsquoBig Bend&rsquo, a 900 change in the river&rsquos course from where it hurtles down and disappears into the Po Tsangpo gorge, which, at 5,030m, is the deepest valley in the world. Defying exploration for centuries, it was not until the late 20th century that the Gorge was finally navigated. Reappearing on the Indian border, the Siang &mdash or the Upper Brahmaputra &mdash crosses the McMahon Line into India&rsquos equally remote East Siang district of Arunachal. The son of Brahma is by now a muscular torrent surging through the Canyon Gorge.