Question

where the Bhrahmaputra river 'U' turn why.

Answer 1

It is not a sudden U-turn of a river. Perhaps you may be knowing that the Indian Sub-continent has two syntaxial bends at its northern extremity – in the NW it is around the Nanga Parbat Peak with deep entrenched Indus River hair pin bend and in the NE it is around Namcha Barwa Peak with deep entrenched Brahmaputra River hair pin bend. These sharp hair pin bends reflect the structural adaptation of river segments as well as deep entrenchment (rapid down cutting) in response to the tectonic upliftment of the terrain (Collision tectonics).

Answer 2

It flows towards east for almost half of its length in mountainous Tibet and then takes a U-turn (photo) to enter plains in Assam. Ultimately, it enters Bangladesh towards South and meets the Ganges before reaching the sea. The place where it takes a U-turn