For more than a month now, the mysterious discoloration of the Siang River has puzzled the people of Arunachal Pradesh and Assam. The turbidity levels of the river are exceptionally high for this time of year. Testing of the blackened waters has indicated that the water is dominated by clay and silt. The heightened turbidity levels (425 nephelometric turbidity units against a December normal of 12-15 nephelometric turbidity units) has made the water progressively unfit for human use and even affected aquatic life, with reports of dead fish coming in. Such a phenomenon has never been witnessed by the local people before and various theories were put forward to explain it.

The Siang river is called the Yarlung-Tsangpo in Tibet, and the Brahmaputra after it merges with the Dibang and Lohit rivers in Assam. Considering the fact that it originates in Tibet, and flows for more than 2,000 km within China, suspicions for the discolouration naturally fell on China. News reports highlighted Chinese plans to divert the waters of the Tsangpo to irrigate parts of the Taklamakan desert in northwest China’s Xinjiang autonomous province. The turbidity was thought to have been a consequence of Chinese construction activity, although there was no evidence to establish this.

The Siang river is usually blue at this time of the year.

Clear and present danger

Our analysis of this phenomenon indicates that while the causes of the Siang’s dark waters are completely natural, the potential danger to downstream inhabitants in India is serious and urgent.

The Gyala Peri mountain lies North of the Yarlung-Tsangpo river and West of the Yigong-Tsangpo river, a tributary. It is here that the Yarlung-Tsangpo cuts the world’s largest gorge between Gyala Peri and the Namcha Barwa mountain. The Yarlung-Tsangpo then merges with the Yigong-Tsangpo and turns 180 degrees South to flow, as the Siang, into Arunachal Pradesh.

On November 18, at 4 am Indian Standard Time, a 6.4 magnitude earthquake struck below the summit of Gyala Peri. For the next 32 hours, this region was swarmed by five earthquakes, all measuring more than 4 in magnitude. A final 4.7 magnitude quake struck the area on November 23. No aftershocks were recorded.

Data source: USGS

Massive landslides on river

The intense vibrations triggered a massive chain of landslides along the Yarlung-Tsangpo river. Several satellites have taken images showing approximately a 100 sq km area of landslides. The volume of material lost is yet unknown. The satellites have captured dust and debris in the air, indicating that landslides continued even 20 days after the earthquakes struck. This landslide area is 400 km upstream from Pasighat, the headquarters of East Siang district, and 200 km from Geling near the international border with China. The affected part of the river is 30 km in length.

The blockage of the Yarlung-Tsangpo river at three locations along a 12-km stretch that falls in Tibet’s Bayi district is a matter of serious concern. Three natural dams have formed one after the other (see image below). While the dams are significantly smaller than the dam that had formed on the Yigong-Tsangpo river in 2000 after a landslide, it is too early to rule out the possibility of these three dams merging and becoming larger.

Based on preliminary calculations from low-resolution data, the current total volume of the Gyala Peri dams is about 1 billion cubic metres, one third the volume of the waters that were dammed on the Yigong-Tsangpo in 2000.

The volume calculations are from coarse data and do not take into account any debris within the dams. However, the dark colour indicates it is not shallow like other parts of the river. Nevertheless, the blockage of river flow by debris is unpredictable as the area may be unstable with falling rocks and mud. Real-time satellite monitoring is required to keep track of how the dams are changing in order to allow areas downstream in Arunachal Pradesh and Assam to prepare accordingly.

This is perhaps the largest landslide of the year and catastrophically larger than the landslide that took place on April 9, 2000, on the Yigong-Tsangpo. That slide was caused by a similar series of events. Two earthquakes, measuring 3.5 magnitude and 4.6 magnitude, struck the Zhamu creek in Tibet that day. The landslide blocked the floodplain river, creating a dam with an area of 2.5 sq km, and 90 meters deep. The natural dam gave way on June 1 that year. Within 12 hours, areas all the way downstream till Assam were flooded. The floods lasted for days. It was the biggest outburst from a landslide-dammed lake ever documented in modern history. The surge swept away large forest areas and several bridges along the Siang. Several elders still remember the intense wave of water that rushed down.

Figures circulated in the aftermath of the incident claimed that 30 people died and more than 50 villages in Arunachal Pradesh and Assam were destroyed. The Arunachal Pradesh government put the damage at Rs 100 crores, but the true cost of this flood is still being felt. Apart from the destruction of bridges, the river deposited sand and debris on terrace farms where the indigenous Adi people cultivated their food crops. Several fields still remain spoilt even 17 years after the incident. People also reported the loss of many Mithun cattle, which is vital to the economy and culture of the Adi people.

Current concerns of the dark colour of the waters of the Siang are well grounded based on past events and must be taken seriously. Usually, the Siang becomes clear blue after November. This prolonged discolouration after the monsoon, coupled with numerous dead fish have alarmed people living along the river.

Immediate action needed

Earlier suspicions and rumours that implicated Chinese dams upstream on the Yarlung-Tsangpo river are incorrect as satellite images distinctly reveal that the origin of the sediments is from the slopes of the Gyala Peri.

In the aftermath of the 2000 flooding, the Indian media had similarly pointed fingers at Chinese dams, assuming that the event was caused by the breach of man-made dams in China. We now know that it was not a man-made dam and the Chinese government failed to warn India of the possible consequences of a breach at that time. Such a warning could have significantly minimised the loss of life and property downstream. The evidence presented above shows that a situation akin to June 2000 cannot be ruled out.

A panel of remote sensing experts, geologists, hydrologists and disaster management experts needs to be set into action right away for the safety of the people. It is also critical for the people of Arunachal to scientifically study the geography and ecology of their region not only prepare for disasters but to develop sustainably.

Chintan Sheth (@tweetweetforest) is with the National Centre for Biological Sciences and Anirban Datta-Roy (@clixbyman) is with Ashoka Trust for Research in Ecology and the Environment.

A version of this piece was first published in the Arunachal Times.