Siang dam's Himalayan water challenge and the cost of development

By Pooja Gupta 

The Siang Upper Multipurpose Project (SUMP), proposed in Arunachal Pradesh's Upper Siang district, is set to become India's largest hydropower project—a nearly 11,000 MW structure conceived to regulate the flow of the Siang River, which becomes the Brahmaputra downstream in Assam. 

It is also one of the most consequential questions facing the Northeast today because it sits at the intersection of three legitimate concerns that do not easily resolve into one another: national water security, the wellbeing of the Adi and Galo communities who call this valley home, and India's strategic position vis-à-vis China's upstream dam-building on the Yarlung Tsangpo.

The case for SUMP begins upstream, in Tibet. China began construction in mid-2025 on the Medog Hydropower Station, a five-dam cascade at the Yarlung Tsangpo's dramatic "Great Bend," with a planned capacity of roughly 60,000 MW—nearly three times that of the Three Gorges Dam. China is not a signatory to any binding international water-sharing treaty with India, and hydrological data-sharing has been inconsistent in the past. For a river system on which nearly a billion people ultimately depend downstream, that combination of scale and uncertainty is a genuine planning concern, not an exaggerated one.

SUMP is essentially expected to serve three purposes: regulating the Siang's flow so that unusual releases from upstream do not translate into flash flooding in Arunachal Pradesh and Assam; establishing India's own substantial water infrastructure and prior-use position on a river without a formal water-sharing treaty; and generating clean power for a northeastern grid that badly needs it, with a share of free electricity promised to the state.

Arunachal Pradesh Chief Minister Pema Khandu has described the project in these terms—as an insurance policy against unpredictability upstream, being developed at a time when India cannot simply wait for a bilateral water-sharing arrangement that may be years away.

The Concerns That Deserve Equal Weight

At the same time, the project's scale is significant enough that concerns raised by local communities cannot be treated as a footnote to the national story. They are a central part of it.

Estimates of how many villages and people would be affected have varied as the project has evolved, ranging from roughly a dozen submerged villages in earlier assessments to newer estimates by the Siang Indigenous Farmers' Forum (SIFF) of around 27 villages and well over 100,000 people whose land, homes or farming livelihoods could be affected in some way. For a community where agriculture is a primary occupation, and where land carries cultural as well as economic value, that is a substantial demand to place on any population.

Consultation has been a genuine point of friction. Village-level agreements have been signed at various stages—most recently in March 2026, when families from Simong and Halleng villages backed the project's preliminary survey stage—and these are a positive indication that engagement is possible. But other agreements have been disputed by residents of the same villages, while the Adi Bane Kebang, the community's apex traditional body, has taken a firmer position against the project in its current form.

The deployment of central security personnel to facilitate survey work in 2025 also drew strong local pushback. It was a reminder that for a project of this scale, the manner of consultation can matter almost as much as its content.

None of this is necessarily an argument against the dam itself. It is an argument for slowing the process enough to ensure that consent is unambiguous and durable, rather than contested at every stage.

What the Valley's Knowledge Systems Bring to the Table

One dimension of this debate deserves far more attention than it usually receives: what the Siang Valley's own accumulated knowledge can contribute to precisely the problems SUMP is intended to address.

The Adi and Galo communities have developed, over generations, a detailed working knowledge of the river system—a winter fishing method called Lipum, built around stone structures placed with an intimate understanding of seasonal flow and sediment behaviour; an elaborate terraced wet-rice cultivation and irrigation system engineered largely by hand; and a documented reliance on more than 250 species of wild plants and animals for food, medicine and cultural use.

A concrete example, recorded in field research on Adi biocultural knowledge, illustrates how specific this knowledge can be. Otek Moyong, an Adi knowledge-holder from the region documented for her expertise in local biodiversity, described how a particular insect found among rocks and moist riverbank sand near the Siang is read as a seasonal signal. If it takes flight around sunset, it is understood to indicate a good rainfall year ahead, prompting farmers to plant taller paddy varieties suited to wetter conditions.

It is a small, precise and locally calibrated forecasting practice, developed long before formal weather monitoring reached the valley. Notably, the insect's habitat has reportedly become vulnerable to disturbance from construction activity along the riverbank.

This is not an isolated example. It is representative of the kind of granular, tested knowledge held across the valley—in the form of seasonal indicators, planting calendars and land-use practices—that formal climate-adaptation planning in the Northeast is only beginning to document.

That knowledge is directly relevant to the climate-risk conversation around SUMP. Independent reviews of the project site have raised concerns about Glacial Lake Outburst Floods (GLOFs) and broader climate-driven instability in the upper catchment. The damage caused to the Tapovan-Vishnugad hydropower project in Uttarakhand after a 2021 glacier-related disaster is often cited as a reminder that Himalayan infrastructure must be designed with genuine climate resilience in mind, not simply structural engineering.

The same unpredictability that makes a regulating reservoir attractive to planners is the unpredictability that Adi communities have spent generations learning to read.

A resettlement and planning process that treats this knowledge as incidental—rather than something worth documenting, consulting and, where possible, integrating into the project's own risk assessments—would overlook a resource that is genuinely useful, not merely symbolically important.

Finding the Balance

None of these considerations points to an easy answer, and it should not. India's interest in building water-regulation capacity on its own side of an unregulated river is reasonable, and the Northeast's power needs are real.

Equally, a project on this scale, in a seismically active and ecologically rich valley inhabited by communities with a deep and specific relationship to the land, deserves a process that treats consent as something to be earned rather than assumed. Displacement must be treated as a cost to be minimised and fairly compensated, not as a formality to be managed.

A more balanced path forward would combine several measures that are not, in practice, in tension with one another: continuing to build India's flow-regulation and storage capacity as a genuine safeguard; pursuing sustained diplomatic engagement with China and establishing consultation mechanisms so that unilateral infrastructure is not the only tool available; ensuring that consent processes with Siang Valley communities are transparent and leave no ambiguity about what has actually been agreed; and formally documenting and drawing on Adi and Galo ecological knowledge as part of the project's own climate-resilience planning, rather than treating it as something the reservoir will simply submerge.

Water security, humanitarian concerns and India's strategic interests in the Northeast are not ultimately competing priorities. A project that succeeds on only one of these fronts—power and flood control without community trust, or community engagement without adequate water-security planning—will not have solved the underlying problem the Siang Valley faces.

Getting SUMP right, rather than merely getting it built, will determine whether it becomes a durable answer to the region's water-security challenge or another source of the very instability it is meant to guard against.

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The author is a Climate Adaptation, Resilience & Energy Transition Professional