A new report by the advocacy group South Asia Network on Dams, Rivers and People (SANDRP) has warned that China's proposed 60,000 MW Medog hydropower project and India's proposed 11,000 MW Siang Upper Multipurpose Project (SUMP) are being planned in one of the world's most geologically unstable and disaster-prone regions, potentially exposing millions of people downstream to serious risks.
The report, titled "The Riskiest Place to Dam a River: Medog, SUMP and the Untamed Yarlung Tsangpo Gorge", has been authored by Parineeta Dandekar with Chintan Sheth. It argues that the combination of seismic activity, landslides, glacial lake outburst floods (GLOFs), extreme sediment flows and climate change makes the Yarlung Tsangpo gorge an exceptionally hazardous location for large hydropower infrastructure.
The report comes amid growing opposition in Arunachal Pradesh to the proposed SUMP, which the state government has presented as a strategic response to China's Medog project on the Yarlung Tsangpo, known as the Siang after it enters India and the Brahmaputra further downstream.
According to the report, China's Medog project, described as the world's largest hydropower project, is expected to have an installed capacity of approximately 60,000 MW. However, Beijing has not made public essential details concerning the project's precise locations, reservoir capacities, water discharge arrangements and operational objectives.
The report says that the lack of transparency is particularly concerning because the project is located close to India's border and could have significant implications for downstream regions in Arunachal Pradesh and Assam.
It notes that China's proposed project is being promoted on grounds of national security, energy production, ecological considerations and international cooperation, but the absence of reliable technical information makes it difficult for downstream countries to assess its potential consequences.
Against this backdrop, Arunachal Pradesh is advancing the proposed 11,000 MW SUMP, which Chief Minister Pema Khandu has described as a water-defence project. The proposed project is expected to involve a dam approximately 280-300 metres high, with a reservoir capacity of 9-10 billion cubic metres.
However, the report questions whether constructing another massive dam downstream is an appropriate response to the risks associated with China's project.
It points out that the proposed SUMP could completely submerge 27 Adi tribal villages and partially affect another 40-45 villages. Thousands of orange and pineapple farms and traditional wet-rice terraces could also be affected, while more than 100,000 indigenous people could face displacement or loss of livelihoods and land.
The report describes the opposition of indigenous communities as a struggle over their land, livelihoods and future, arguing that the concerns of the Adi people cannot be overlooked in the pursuit of strategic and hydropower objectives.
Beyond the geopolitical concerns, the report identifies the geographical and geological characteristics of the Yarlung Tsangpo gorge as a major reason for questioning both projects.
The gorge, located around the Namcha Barwa or Eastern Himalayan Syntaxis, is among the deepest river gorges in the world. The river makes a dramatic horseshoe-shaped turn around Namcha Barwa before entering India, descending approximately 2,000 metres over a relatively short stretch.
According to the report, this region experiences some of the world's highest rates of tectonic uplift, rock erosion and river sediment transport. It is also highly vulnerable to earthquakes, landslides, avalanches and glacial lake outburst floods.
The report cites the 1950 Assam earthquake, which measured 8.6 on the Richter scale, as evidence of the region's seismic vulnerability. It also refers to research documenting more than 200 earthquakes of magnitude above four between 1950 and 2018 in the gorge region.
The sediment carried by the river is another major concern. According to the report, hydrological measurements show that the Yarlung Tsangpo carries approximately 15-30 million tonnes of sediment annually at Nuxia/Pai on the Tibetan plateau. By the time it reaches Pasighat in Arunachal Pradesh, the annual sediment load rises to approximately 210-250 million tonnes.
Studies cited in the report estimate that 61-74 per cent of the sediment reaching Pasighat originates from the relatively small gorge region. Although this area constitutes less than three per cent of the Brahmaputra basin, it contributes approximately 45 per cent of the sediment reaching Bangladesh through the river system.
The report warns that large storage dams could substantially alter this natural sediment movement. Such dams can trap most of the sediment flowing into their reservoirs, affecting downstream river ecology, agriculture and the stability of river channels.
It cites the example of the Tehri Dam, which reportedly retains approximately 95 per cent of the sediment it receives.
If the proposed SUMP traps large quantities of sediment arriving from upstream, its reservoir capacity and ability to accommodate floodwaters could progressively decline. The report also warns that reduced sediment flows downstream could increase riverbank erosion and affect communities dependent on the river's natural sediment supply.
The report further draws attention to a series of major landslides and river-blocking incidents in the Yarlung Tsangpo gorge, demonstrating the scale of natural hazards that could affect projects in the region.
The report recalls that the flood swept away forest areas and several bridges. It cites accounts of approximately 30 deaths and destruction affecting more than 50 villages in Arunachal Pradesh and Assam.
Another major series of incidents occurred in 2017, when landslides and rock-ice collapses in the Sedongpu valley partially blocked the Yarlung Tsangpo. The Siang river in India subsequently turned into a dark, muddy flow, affecting fish and local drinking-water infrastructure.
In October 2018, two successive rock-and-ice avalanches completely blocked the main river channel, creating temporary barrier lakes. The resulting changes in water levels affected downstream flows in Arunachal Pradesh before the blockages were breached and surges of water and debris moved towards Assam.
A further major rock-and-ice collapse in March 2021 again blocked the river.
The report says these incidents demonstrate how landslides, glacier collapses, river blockages and sudden floods can combine to create disasters across international boundaries.
It also highlights the impact of climate change, noting that the Tibetan plateau is warming at approximately twice the global average rate. Rising temperatures are accelerating glacier retreat and permafrost degradation, increasing the potential for landslides, avalanches, glacial lake outburst floods and extreme river flows.
The authors warn that these developments could make the region increasingly unpredictable for large hydropower projects.
A major concern raised in the report is the absence of an effective mechanism for sharing hydrological and disaster-related information between China and India.
The existing memorandum of understanding on hydrological data sharing between the two countries expired in June 2023 and has not been formally renewed. Although China agreed in principle in 2025 to share information on a limited emergency and humanitarian basis, the report says that a reliable institutional arrangement is still lacking.
It recalls that during the 2017 Doklam standoff, China withheld hydrological information on the Brahmaputra and Sutlej rivers for the entire flood season. It also points to the lack of advance warnings during several major landslide-related incidents in the Yarlung Tsangpo basin.
The authors argue that the absence of reliable information-sharing arrangements, combined with the possibility of sudden dam releases and natural river blockages, could significantly increase the risks for downstream communities.
The report also questions the flood-control rationale of SUMP. It argues that a large storage reservoir intended to generate electricity would need to retain water and therefore could not remain empty at all times to accommodate sudden floods originating upstream.
If the reservoir's capacity to absorb floodwaters is reduced by sediment accumulation, the consequences of an upstream disaster could be particularly serious.
According to the report, the proposed SUMP could face extreme flows resulting from upstream dam releases, heavy precipitation or natural disasters. If the reservoir loses a substantial part of its flood-storage capacity because of sediment accumulation, its ability to perform its intended flood-control function could be compromised.
The authors conclude that China's Medog project poses a serious geopolitical and environmental challenge, particularly because of Beijing's lack of transparency. However, they argue that India's decision to construct a massive storage dam downstream could create an additional and potentially permanent vulnerability.
"India is responding to an external upstream hazard by building a permanent domestic structural vulnerability," the report states.
It adds that the risks associated with SUMP cannot be adequately addressed simply by presenting it as a strategic response to China's project.
The report calls for China to disclose essential technical information about the Medog project and share it with India and Bangladesh. It also recommends a trilateral basin consultation and a joint transboundary environmental impact assessment involving the countries affected by developments on the river.
Among its other recommendations, the report calls for India to establish an independent earth-observation and early-warning system for the Yarlung Tsangpo basin, using satellite capabilities to monitor landslides, seismic activity, glacial changes and potential river blockages.
It also recommends installing monitoring systems along the border at Gelling, Bishing and Tuting, alongside satellite-based surveillance and scientifically reviewed assessments of geological and hydrological risks.
Equally important, the authors emphasise that the concerns of indigenous communities in Arunachal Pradesh must be taken seriously before proceeding with the proposed SUMP.
The report concludes that the combination of geological instability, climate change, inadequate information-sharing arrangements and the absence of effective disaster-warning mechanisms makes the Yarlung Tsangpo-Siang region one of the most dangerous locations in the world for constructing mega hydropower projects.
It says that the risks associated with the proposed Indian project could outweigh its promised benefits, particularly when the potential consequences for downstream communities, river ecosystems and livelihoods are taken into account.
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