THE ecology of Assam is characterised by a large network of rivers and tributaries originating from the hills of Arunachal Pradesh, Meghalaya, Nagaland, Mizoram, Manipur, and the neighbouring country of Bhutan, which join the Brahmaputra and Barak rivers to form a highly dynamic and complex river system. The Brahmaputra and its tributaries, together with the Barak river system, inundate large areas of the plains of Assam almost every year during the monsoon.