The largest tributary of the Brahmaputra River, the Subansiri River has observed the discharge of 18,799 cubic metres per seconds of water. It contributes 7.92% of the Brahmaputra's total flow. Originated from the Himalaya Mountains, it flows towards the East and the South East portion of the state and then flows to the Assam Valley. Finally, it joins to the Brahmaputra River in Lakhimpur district.
The Subansiri River descends through verdant rainforests in Arunachal Pradesh, draining the waters beyond the Great Himalayan Range from Tsona Dzong up to the Great Bend of the Tsangpo in Tibet. It slices through the Eastern Himalayas to flow into India at Asaphila then on through the rugged isolated hinterlands of Arunachal. It joins the mighty Brahmaputra at the mystic confluence of Majuli Island, which is noted as the largest inhabited river island in the world.
Subansiri River flows through the difficult terrain with vertical rock faces. This is one of the fast-flowing and adrenal-pumping rivers set in a scenic breathtaking environment. Near the Subansiri River, there are the dense hilly population and forming the different districts in Arunachal Pradesh. Lower down, the friendly hill people with their unique traditions and customs developed over a millennium await with trepidation the inevitable changes rapidly encroaching upon them.
A 135 meter high hydroelectric dam is constructed to provide the hydro electric power supply in the adjoining villages and also for Tibet and Bhutan. This river is specially known as the "future powerhouse". According to a 2001 study of the Central Electricity Authority, (CEA) of India, there are 168 hydro projects and a potential of 63.328 MW have been identified in the Brahmaputra river basin. This includes 22 projects having potential of 15,191 Mega Watt in the Subansiri River. The 200 Mega Watt Lower Subansiri hydel power project had proposed to come up at Gerukamukh on the Assam - Arunachal Pradesh border. It is the first large hydroelectric power project to be taken up in the Subansiri River basin.