Question

Which mountains demarcate the deep plains of brahmaputra?

Answer 1

Brahmaputra is one of the mighty rivers of India.The Brahmaputra river originates from Bhagirath Glacier and flows through Himachal,Arunachal pradesh,Assam,Bangladesh.When snow melts in summerin Himalayas ,the
river Brahmaputra river is flooded.Thus,the correct  answer of the question is 
that "The great Himalayas" are the mountain range which demacate river 
Brahmaputra.
During the months July to sebtember the river Brahmaputra is mostly flooded.every year.The water of river Brahmaputra is equally shared by three countries i.e.India,Bangladesh and China.
The meaning of Brahmaputra is "Son of Lord Brahma".

Answer 2

The Himalayan ranges demarcate the deep plains of Brahmaputra River.

However, the local name differs and as per the physiography of Brahmaputra plains, the hills of North Cachar and Karbi Anglong and the ones that are adjacent to Guwahati together with Garo and Khasi mountains demarcates it from south plateau.