Causes behind Brahmaputra turning black could be natural:

A preliminary study has found that the water of the Siang river, which originates in southern Tibet and becomes the Brahmaputra upon entering Assam through Arunachal Pradesh, is turning black due to a recent earthquake in the region, Union minister Arjun Ram Meghwal has said.
"We have been getting reports of pollution in the river and its water turning black. The Central Water Commission (CWC) has started looking into the matter recently and people have been sent to the internal areas," the Union minister of state for water resources said here yesterday.
He added that according to the preliminary findings, the path of the river was temporarily obstructed after an earthquake in Tibet on November 17.

"There is a strong possibility that the colour (of the river's water) has changed due to natural reasons," the minister said.
Congress MP Ninong Ering, representing the Arunachal East constituency in Parliament, had recently written a letter to Prime Minister Narendra Modi in this regard, noting that the water of the river changing its colour was an unusual phenomenon in the winter months.
In his letter, Ering had also pointed out that there were reports of China constructing a 600-km tunnel in its Yunnan province to divert the Siang water to the Taklamakan desert through the Xinjiang province, even though the neighbouriong country had denied any such development.

He had mentioned in the letter that the reports of the Tsangpo (Siang or Brahmaputra) being diverted had appeared a few months back.

Although China had denied it, the matter remained an area of concern for India, the Congress MP had mentioned in his letter to the prime minister.
"After these reports appeared, the water of the river Siang has turned muddy and slushy. It has been two months since the Siang turned black and contaminated. It is an unusual phenomenon.
"The reason for the river getting dirty is unknown. I have already put up questions for discussions in Parliament under rule 377. But since the House is not in session, I am requesting you to use your good office to seek the reason for the river turning muddy in this season, when the water is usually crystal clear," Ering had written in his letter to Modi.

He had also pointed out that there could be no reason for the river getting contaminated in the month of November other than a possible heavy land excavation on the Chinese side, adding that it had to be verified by an international team.
Certain reports claim that the water of the river have become unusable and that a number of fish had died in the river in the recent past.