The Lhasa expedition in 1904 would fain have traced the river to the plains had not the Government interposed a veto. In the years that followed, the source of the Tsangpo was discovered by Captain Rawling. In 1911 the Abor expedition increased our knowledge of the course of the Brahmaputra right up to the skirts of the main range. The problem now was not the linking up of the Tsangpo and the Brahmaputra, but what happened to the river in the hairpin bend between Pemakochung and its debouchment in the Abor valleys. The elevation of the stream at the point where the main road to Lhasa crossed it was in the neighbourhood of 12,000 feet. From there as far as Pemakochung we knew that there was no very great loss in altitude, but when the Brahmaputra appeared in the Abor foothills it was only between 1,000 and 2,000 feet above the sea. The stretch of unknown course was perhaps 200 miles, and in that section the river broke through the main range of the Himalaya. It was possible—nay, it was probable—that somewhere in those gorges, which Kinthup had thought impassable, lay hidden the most tremendous waterfall in the world.

The secret of the Brahmaputra gorges was one of the topics that most fascinated geographers between the years 1904 and 1913. In that latter year the mystery was solved, and the ignotum proved not to be the magnificum. This is the story of the solution.

The course of the Brahmaputra through Assam is roughly from north-east to south-west, but at a place called Sadiya the main stream, there known as the Dihang, turns sharply to the north. At that point, too, it receives an important tributary on its left bank called the Dibang. During the winter of 1912-13 Captain F. M. Bailey, an officer of the Indian Political Service, was employed by the Government to survey the Dibang basin, while another party had gone through the Abor country to survey the Dihang. Early in 1913 Captain Bailey and Captain Morshead of the Royal Engineers collected what stores they could and started off from the village of Mipi on the upper waters of the Dibang. Their aim was to cross into the Dihang valley, and to follow the river upstream to the Tibetan plateau. Captain Bailey had been with the Lhasa expedition, and had had a long record of exploration in different parts of Tibet, so he had all the qualifications needed by the pioneer. But his party was imperfectly equipped, since it started more or less on the spur of the moment, and had no time to obtain proper stores from India. He trusted to the prestige won by the Abor expedition, and his experience of the ways of the Tibetans, to furnish him with coolies and local supplies.

The reader's attention is now prayed for the map. The first business was to cross the high passes separating the Dibang from the Dihang. The weather proved abominable, and for part of the route only half rations could be issued. As they descended into the valley of the Dihang they found once more cultivation and villages, and they were able to supplement their stores by shooting game, especially pheasants, which teemed by the roadside. It was necessary to establish touch with the Abor Survey party lower down the river, and accordingly they had to halt for some days. At a place called Kapu they managed to take the altitude in the river bed, and found the height above sea level to be 2,610 feet—an important result, for they were able to take no other observation at water level below the main gorges.

These foothills of the Himalaya were inhabited chiefly by savage tribes akin to the Abors, who were known generically as Lopas. But as the expedition advanced up the river they came to the country of the Pobas, who were under Tibetan influence. At Lagung, which is about the centre of the hairpin bend, the course of the river turned west. It might have been possible for them to have followed it some thirty miles farther, but they were pressed by a Poba official, with whom they made friends, to go north-east into the absolutely unknown country of Po-me, which would enable them to make a circuit and reach Gyala at the head of the gorges. Captain Bailey considered that it would be easier to explore the gorges by going downstream.

On 21st June they crossed a pass of over 13,000 feet into the valley of the river known as the Po-Tsangpo, an affluent of the Brahmaputra. It was a stream 80 yards wide, and of such rapidity that its current was one whirl of foam. The natives were in great fear of the Chinese, and it was necessary to go boldly to Showa, the capital, where a letter could be received from the Abor Survey party vouching for their respectability. The Chinese had burned the place, killed the chief, and decapitated the council, and the inhabitants looked askance at the travellers because of the Chinese writing on a tablet of Indian ink which they carried. After three days, however, a letter arrived from the Abor party, which persuaded them that Captain Bailey and Captain Morshead were at any rate servants of the English King.

The explorers now moved north-eastwards down the Po-Tsangpo, finding great difficulty in crossing the tributaries, where the bridges had mostly been destroyed. It was a beautiful land, bright with primula, iris, and blue poppy, and the roads were lined with raspberries. They were now leaving the Po-me country and travelling among a more civilized type of Tibetan, who wore hats like clergymen, made out of yak's hair. After crossing a pass of over 15,000 feet they returned to the main stream of the Tsangpo. This country was under the charge of Tzongpen of Tsela, who came to meet the travellers—an urbane gentleman whose son was at Rugby and a promising cricketer.

They were now on the Tsangpo above the mysterious gorges. They had left behind them the hot valleys of the lower stream and found a dry Tibetan dale, where the chief crops were barley and buckwheat. The river was broad and slow, at one point stretching into a lake 600 yards wide, and its altitude was 9,680 feet. The problem was now to follow it down from that point to the point of their last observation, where the altitude was only 2,610 feet. Somewhere in the intervening tract of gorge it must make the enormous descent of over 7,000 feet.

The first stage was the twenty-two miles down to Gyala, which had been visited in 1878. The stream was in flood owing to melting snows, and the water-side track was difficult. Four days' march below Gyala they reached Pemakochung, the limit of Kinthup's exploration. So far they had passed various small rapids, but nothing in the nature of a fall. A mile below Pemakochung they came on Kinthup's cascade. It proved to be only some 30 feet high and not vertical.

The road now became extraordinarily intricate. Great spurs ran down to the river and blocked the glen, and it was necessary to cut paths through dense forest and thickets of rhododendron to surmount them. There was no track of any kind, and the tributaries descending from the adjacent glaciers were often hard to cross. They ran short of food, and could get no reliable information as to the possibility of their descending the stream. Captain Morshead and the coolies accordingly returned to Gyala, and Captain Bailey, with one man and fifteen pounds of flour, attempted to descend the Tsangpo by the route which a party of Monbas was said to have recently taken. He found the Monbas, but they were wild and suspicious and far from helpful. They refused to take him to their village, and declined to show him the road round the difficult cliffs. Apparently they considered that a traveller who had only one servant, and who carried most of his baggage himself, must be a person of small importance and not worth troubling about. He managed, however, to pick up from them certain news about the lower valley.