The largest river in Tibet is the Erechumbu, or Yaru-tsangbu; _tsangbu_ means river, and is often alone used for this whole name. It rises in the Tamchuk range, at the Mariam-la pass in Nari, 60 miles east of Lake Manasarowa, the source of the Sutlej; it flows a little south of east for about seven hundred miles, through the whole of Southern Tibet, between the first and second ranges of the Himalayas, as far as long. 90° E. Its tributaries on the north are numerous, and among them the Nauk-tsangbu and Dzangtsu are the largest. The volume of water which flows through the mountains into Assam by this river, is equal to that by the Indus into Scinde. The disputed question, whether the Yaru-tsangbu joins the Brahmaputra or Irrawadi, has been settled by presumptive evidence in favor of the former, but a distance of about 400 miles is still unexplored;[136] the fall in this part is about 11,000 feet, to where the river Dihong has been traced in Assam. This makes the Brahmaputra the largest and longest river in Southern Asia; its passage into Assam is near 95° E. longitude. The eastern part of Tibet, beyond this meridian, is traversed by numerous ranges of lofty mountains, having no separate names, the direction of which is from west to east, and from northwest to southeast. From these ranges, lateral branches run out in different directions, containing deep valleys between them. In proportion as the principal chains advance towards the southeast they converge towards one another, and thus the valleys between them gradually become narrower, until at last, on the frontiers of Yunnan and Burmah, they are mere mountain passes, whose entire breadth does not much exceed a hundred miles, having four streams flowing through them. In fact, Tibet incloses the fountain heads of all the large rivers of Southern and Eastern Asia. The names and courses of those in Eastern Tibet are known only imperfectly from Chinese maps, but others have described them after their entrance into the lowlands.