At Pasighat, the Siang river spreads out to flow downstream into Assam and becomes the Brahmaputra.
An 82-year-old retired government employee, Kakut Tayeng has lived by the blue waters of Siang river in Arunachal Pradesh all his life. So when about two months ago, the water of the started turning black, muddy and turbid, Mr Tayeng was in for a shock.
Known as the Yarlung Tsangpo River in China, the river enters India at Shuomatan Point, becoming the Siang. At Pasighat, the Siang spreads out to flow downstream into Assam and becomes the mighty Brahmaputra.
Around October this year, the waters of the river started turning black. Some local residents believe that China may have something to do with this. One of the theories is that the Chinese government's purported plan to build a tunnel to divert water from the Yarlung Tsangpo in southern Tibet to the parched Taklamakan desert area in the province of Xinjiang may have triggered the change.
"This river was so beautiful; there were many fishes around. Since 1947, the river has been spreading out in winter. Never before have we seen such blackish water," Mr Tayeng said.
Both Assam and Arunachal Pradesh have raised alarm after initial tests reports showed high turbidity but the centre is yet to decisively address the concerns.
"China is still denying that they are not doing anything. In 1962, the Chinese said '
Hindi-Chini bhai bhai' and then they came shooting at us. The same thing is happening. I am really worried about the central government. Are they really thinking about the North-East," the local MP, Ninong Ering, told NDTV.
Some others think the change in the river maybe one of the aftereffects of the recent earthquake in Tibet.
A sample tested on November 27 showed a Nephelometric Turbidity Unit - a measure of the concentration of suspended particulates in a liquid - of 425. The permissible turbidity for drinking water is 5 NTU.
"I do not see fishermen here. The aquatic life is all gone," wildlife and biodiversity expert P Rings said. "There is a wildlife sanctuary 3 km downstream and we used get migratory birds every year. Now I can no longer see those migratory birds," he added.
Residents of Pasighat resent New Delhi's inaction. The deteriorating state of the river not only threatens the region's biodiversity but also the lives of thousands of residents of Arunachal Pradesh.