For about 250 m. below Kyi-chu to a point about 20 m. below the great southerly bend (in 94° E. long.) the course of the Brahmaputra has been traced by native surveyors. Then it is lost amidst the jungle-covered hills of the wild Mishmi and Abor tribes to the east of Bhutan for another 100 m., until it is again found as the Dihong emerging into the plains of Assam. About the intervening reaches of the river very little is known except that it drops through 7000 ft. of altitude, and that in one place, at least, there exist some very remarkable falls. These are placed in 29° 40' N. lat., between Kongbu and Pema-Koi. Here the river runs in a narrow precipitous defile along which no path is practicable. The falls can only be approached from below, where a monastery has been erected, the resort of countless pilgrims. Their height is estimated at 70 ft., and by Tibetan report the hills around are enveloped in perpetual mist, and the Sangdong (the "lion's face"), over which the waters rush, is demon-haunted and full of mystic import. Up to comparatively recent years it was matter for controversy whether the Tsanpo formed the upper reaches of the Dihong or of the Irrawaddy. From the north-eastern extremity of Assam where, near Sadya, the Lohit, the Dibong and the Dihong unite to form the wide placid Brahmaputra of the plains--one of the grandest rivers of the world--its south-westerly course to the Bay of Bengal is sufficiently well known. It still retains the proud distinction of being unbridged, and still the River Flotilla Company appoints its steamers at regular intervals to visit all the chief ports on its banks as far as Dibrugarh. Here, however, a new feature has been introduced in the local railway, which extends for some 80 m. to Sadya, with a branch to the Buri Dihing river at the foot of the Patkoi range. The Patkoi border the plains of Upper Assam to the south-east, and across these hills lies the most reasonable probability of railway extension to Burma.
The following are the "lowest level" discharges of the principal affluents of the Brahmaputra in Upper Assam, estimated in cubic feet per second:--
Lohit river, 9 m. above Sadya 38,800 Dibong, 1 m. above junction with Dihong 27,200 Dihong " " Dibong 55,400 Subansiri 16,900