The Yarlong Tsangpo, Tibet's principal waterway, is the upper half of India's great Brahmaputra River. It is 2,900 kilometers ( 1,800 miles) long from its source in western Tibet to its mouth in Bangladesh. Tsangpo, appropriately, means ' mighty'. The Yarlong Tsangpo is the highest river in the world, with an average altitude of 4,000 meters ( 13,000 feet). It flows from west to east parallel to the Himalayas Mountains on their northern side.

Cascading from a high glacier, gathering snow water, the Yarlong Tsangpo enters a long, flat valley above Shigatse as a typical braided river winding among sandbanks. It flows through temperate, fertile central Tibet to Tsedang, where the river nurtured Tibet's first civilization. Along the 650 kilometers ( 400 miles) of its middle reaches, a wide, navigable channel, shifting with the seasons, is plied by river craft: passenger and cargo boats driven by converted tractor engines; oblong, wooden box-boats; and round coracles made of yak hide and willow boughs.

In eastern Tibet, the river hooks dramatically around the snow-capped massif of Namcha Barwa, 7,756 meters ( 25,447 feet) high, and turns south to India, where it is renamed Brahmaputra ( son of Brahma, the Creator). Here it turns west, crossing the plain of Assam for hundreds of miles, parallel to its course through Tibet but in the opposite direction and on the south side of the Himalayas. Its waters finally merge with those of the Ganges in a vast delta flowing south to the Bay of Bengal.

For a long time the Yarlong Tsangpo's source remained a mystery for Western geographers. An Indian named Kinthup, one of the intrepid surveyor -spies employed by the British in the 1800s, first traced it to the sacred Kailash Range of western Tibet. In 1904-5, after Colonel Younghusband had forced Tibet into cordial relations with British India, four British army surveyors recorded their journey up the river from Shigatse through formidable terrain and hardships to confirm its source in a huge glacier southeast if Mt Kailash and Lake Manasarovar.

Even more perplexing was its course through the eastern Himalayas, once people realized that the Yarlong Tsangpo and the Brahmaputra were one and the same river. They knew only that it entered the mountains on the north at 3,600 meters ( 12,000 feet), and that it dropped an astounding 3,350 meters ( 11,000 feet) before emerging in India on the south. Excited geographers predicted an immense, hidden waterfall. Explorers finished their work in 1924 but found no falls higher than nine meters ( 30 feet). Instead they discovered a series of incredible rapids and cascades, whose violent waters raced ten meters( yards) per second through towering cliffs and gorges, eroding the riverbed deeper and deeper into the limestone rock.