Vajravarahi, the river connecting two rivers
Kamal Lodaya

Brahmaputra is the Son of Brahma. It is rare for a river to have a masculine name. In terms of its discharge, the water it carries, Brahmaputra is the largest river in India and one of the largest in the world. It has the largest river island, Majuli in Assam.
The river was said to originate from a vast lake in the Himalaya called Brahma Kund (or Parashurama Kund). There is a lake by this name on the Lohit river. At Sadiya in Assam, the huge Lohit river coming from the east, meets the Siang (or Dihang) river coming from Arunachal Pradesh in the north. There the Lohit indeed appears bigger. Going east, it could be seen at the Parashurama Kund that the Lohit does not originate from there, but comes from further east. The great Assam earthquake of 1950 broke up the Kund. Now it is very clear that the river is coming from further east. Today we know that the Lohit comes from eastern Tibet. It is the Siang which comes from a longer distance, so it is the main stream of the Brahmaputra.
The Yarlung Tsangpo (Zangbo in Tibetan) is the longest river in Tibet, flowing from the west near Manasarovar eastwards across the plateau. It is said to be the highest river in the world. The Tibetans do not seem to have been curious where the Zangbo went. It could have become the Irrawaddy river which flows through Myanmar (Burma). Today we know the Zangbo is not connected to the Irrawaddy, which originates from the Himalayas in Myanmar.
Today we know the Tsangpo turns south and flows through a 500 kilometre gorge, possibly the longest in the world, now called the Yarlung Zangbo Grand Canyon. It drops from an altitude of 2900 metres down to 1500 metres in the Upper Gorge, and then down to 660 metres in the Lower Gorge. This river is called the Siang when it enters India at Gelling village in Arunachal Pradesh.
In this age of satellite pictures, it is hard to believe that until the 20th century it was not known what the exact course of the Tsangpo or the Brahmaputra was. One could not even fly an aeroplane above the river. The region of the gorges is high in altitude and densely forested. There is no human settlement.

Experiments in geography
Lepchas are the native people of Sikkim. One such person, Kinthup, was assigned by Captain Harman of the Survey of India in 1880 to solve this riddle. He had been hired by a Buddhist lama to go to Tibet. At a prearranged time, he was to put 500 specially marked logs into the Tsangpo river at Lhasa in Tibet. A few days later Captain Harman's men would watch for those logs to come down the Siang in Assam. Unfortunately the lama gave Kinthup as a servant to a Tibetan abbot. Later Kinthup managed to get freedom from his lord. In 1884 he arranged a new time schedule, wrote to Harman, and put the logs into the Tsangpo river at Lhasa. Sadly Harman died and no one read Kinthup's letter or watched for the logs.
Kinthup travelled east and climbed high up a mountain in eastern Tibet, possibly one of the slopes around Namcha Barwa, tracking the Tsangpo. From there he could identify settlements on the Siang side. He thought the river would go around Namcha Barwa and fall down a huge waterfall to become the Siang. 
The Dalai Lama of Tibet did not like someone giving information about their country to the British. He asked them not to spy on Tibet. In 1904 the British sent a military expedition from Sikkim to Lhasa to establish their dominance in Tibet. They succeeded in frightening the Dalai Lama. 
In 1911-12, the British started sending army expeditions up the Assamese tributaries of the Brahmaputra into Arunachal Pradesh to find out which one connected to the Tsangpo. They terrorized the tribes living on these tributaries, sometimes burning their villages, to find information about the river.
One of these expeditions measured the height of Namcha Barwa to be almost 7800 metres. Its sister mountain Gyala Peri is almost 7200 metres. The 32 highest mountains in the world are in the Himalayas, Namcha Barwa is the easternmost of them.

River goddess
In 1913, British army officer Frederick Bailey broke away from the military expedition to spend time with Adi tribes. He stayed for a month at Mipi village in Arunachal Pradesh. He learnt from village headman Gyamtso and Buddhist monks that they knew about the great bend of the Tsangpo river around Namcha Barwa mountain. According to them, the looping figure of the Tantric Buddhist goddess Vajravarahi (the shape of her body) corresponds to the geography of this great bend. There is no huge waterfall along the river but there are small waterfalls.
With their blessings he set out with Henry Morshead up the Siang. They reached the Tsangpo gorge and travelled through a lot of it. Although they did not cover all of the gorge, their photographs and other details confirmed to geographers that the Tsango-Siang connection had been made. Picture shows a parasitic flower in the Siang valley.
In 1987 the highest waterfall of the gorge, about 30 metres, was photographed from a Chinese military helicopter. It is only in 2002 that an international team was successful in kayaking all the way down the Tsangpo gorge. The picture of the waterfall is from this team.
From various sources