linguistic:paraphrasing
Arunachal Pradesh

The Yarlung Tsangpo leaves the part of Tibet to enter Indian state of Arunachal Pradesh, where the river is called Siang. It makes a very rapid descent from its original height in Tibet and finally appears in the plains, where it is called Dihang. It flows for about 35 km (22 mi) southward after which, it is joined by the Dibang River and the Lohit River at the head of the Assam Valley. Below the Lohit, the river is called Brahmaputra and Doima (mother of water) and Burlung-Buthur by native Bodo tribals, it then enters the state of Assam, and becomes very wide—as wide as 20 km (12 mi) in parts of Assam.

The reason for such an unusual course and drastic change is that the river is antecedent to the Himalayas, meaning that it had existed before them and has entrenched itself since they started rising.