Save Siang

Why store water on the Siang?

China is building a huge dam upstream on the same river. A reservoir on the Siang is meant to guard against a sudden release of water from it and keep a steady flow in the dry months.

What is China building upstream of the Siang?

The project is on the lower Yarlung Tsangpo River in Tibet.

Screenshot of Chinese state television (CCTV) showing officials applauding on a stage in front of a red banner for the groundbreaking ceremony of the lower Yarlung Tsangpo hydropower project, July 2025, Nyingchi, Tibet.

Construction was announced in July 2025

On 19 July 2025, Chinese Premier Li Qiang announced the start of construction at a groundbreaking ceremony in Tibet.

A deep mountain gorge with a river winding through it.

Five power stations are planned

The project consists of five hydropower stations arranged along the river. Together, they are intended to store water and generate electricity. China could also divert the river’s water.

Google Maps satellite image of Tibet, Arunachal Pradesh and Assam, with drawn lines showing the Yarlung Tsangpo flowing into the Siang, which joins the Dibang and Lohit and becomes the Brahmaputra.

How is this connected to the Siang?

The Yarlung Tsangpo flows from Tibet into Arunachal Pradesh, where it is known as the Siang. After joining the Dibang and Lohit, it flows through Assam as the Brahmaputra.

A river flowing out of a dark mountain gorge into forested hills.

Where does the Siang’s water come from?

Across the year, about 65% of the water in the Siang flows in from Tibet, across the border. In winter, when the river is low, that share rises to about 84%.

A huge muddy flood filling a mountain valley beside a village on green terraces.

Can the river be used as a weapon?

A sudden release of water upstream, or a dam failure, could send a huge surge down the Siang valley. If water is diverted instead, winter flows at the border could drop by up to 85%.

A full, flowing river beside a riverside village, with a man casting a fishing net, two women carrying cane baskets and a wooden boat on the sand.

What does this mean downstream?

India has raised concerns about the project’s effects downstream. How will water be stored and released, and what could that mean for the Siang’s flow?

See the slides (7)

What is China building upstream of the Siang? The project is on the lower Yarlung Tsangpo River in Tibet. Photo of the turquoise Yarlung Tsangpo River bending through a rocky canyon in Nyingchi, Tibet.

Construction was announced in July 2025. On 19 July 2025, Chinese Premier Li Qiang announced the start of construction at a groundbreaking ceremony in Tibet. Screenshot of Chinese state television (CCTV) showing officials applauding on a stage in front of a red banner for the groundbreaking ceremony of the lower Yarlung Tsangpo hydropower project, July 2025, Nyingchi, Tibet.

Five power stations are planned. The project consists of five hydropower stations arranged along the river. Together, they are intended to store water and generate electricity. China could also divert the river’s water. A deep mountain gorge with a river winding through it.

How is this connected to the Siang? The Yarlung Tsangpo flows from Tibet into Arunachal Pradesh, where it is known as the Siang. After joining the Dibang and Lohit, it flows through Assam as the Brahmaputra. Google Maps satellite image of Tibet, Arunachal Pradesh and Assam, with drawn lines showing the Yarlung Tsangpo flowing into the Siang, which joins the Dibang and Lohit and becomes the Brahmaputra.

Where does the Siang’s water come from? Across the year, about 65% of the water in the Siang flows in from Tibet, across the border. In winter, when the river is low, that share rises to about 84%. A river flowing out of a dark mountain gorge into forested hills.

Can the river be used as a weapon? A sudden release of water upstream, or a dam failure, could send a huge surge down the Siang valley. If water is diverted instead, winter flows at the border could drop by up to 85%. A huge muddy flood filling a mountain valley beside a village on green terraces.

What does this mean downstream? India has raised concerns about the project’s effects downstream. How will water be stored and released, and what could that mean for the Siang’s flow? A full, flowing river beside a riverside village, with a man casting a fishing net, two women carrying cane baskets and a wooden boat on the sand.

How can 11,200 MW hold against 60,000 MW?

A common question about SUMP – Siang Upper Multipurpose Project. Here is the simple answer.

The inside of a hydropower station: a row of large generator units in a bright machine hall.

Megawatts measure electricity, not water

MW tells you how much electricity a project can generate. It says nothing about how much water it can hold. Power depends on two things: how far the water falls (the head) and how much water flows (the discharge).

Photo of the Yarlung Tsangpo river in Tibet, with two drawn arrows to scale: a long one labelled Medog, China, about 2 km, and a short one labelled SUMP, about 280 to 300 metres.

Why is China’s number so big?

Because of the drop. At China’s Medog dam the expected head, the distance the water falls, is about 2 km. At SUMP the estimated head is about 280 to 300 metres. That is why the power numbers are so different. It is not because Medog holds more water.

Aerial image of an ordinary, unnamed dam holding a reservoir in green hills, with two drawn bars to scale labelled Medog, China, about 5.5 and SUMP, about 9.2 billion cubic metres.

The right comparison is water stored

In a sudden release of water, what matters is how much water can be stored, and MW cannot tell you that. The estimated storage capacities: Medog about 5.5 billion cubic metres; SUMP about 9.2 billion cubic metres, about 1.7 times more.

An ordinary concrete dam holding back a full reservoir between forested hills.

Power is not protection

Higher generation capacity gives no protection against a sudden release or diversion of water by China. SUMP’s proposed storage is meant to guard against sudden releases. That is why storage is SUMP’s main purpose and power is secondary. These are estimates, and SUMP is still at the PFR (study) stage.

See the slides (5)

How can 11,200 MW hold against 60,000 MW? A common question about SUMP – Siang Upper Multipurpose Project. Here is the simple answer. Two young Adi men in traditional dress on a stony riverbank, looking at a phone together, with a wide green river and forested hills behind.

Megawatts measure electricity, not water. MW tells you how much electricity a project can generate. It says nothing about how much water it can hold. Power depends on two things: how far the water falls (the head) and how much water flows (the discharge). The inside of a hydropower station: a row of large generator units in a bright machine hall.

Why is China’s number so big? Because of the drop. At China’s Medog dam the expected head, the distance the water falls, is about 2 km. At SUMP the estimated head is about 280 to 300 metres. That is why the power numbers are so different. It is not because Medog holds more water. Photo of the Yarlung Tsangpo river in Tibet, with two drawn arrows to scale: a long one labelled Medog, China, about 2 km, and a short one labelled SUMP, about 280 to 300 metres.

The right comparison is water stored. In a sudden release of water, what matters is how much water can be stored, and MW cannot tell you that. The estimated storage capacities: Medog about 5.5 billion cubic metres; SUMP about 9.2 billion cubic metres, about 1.7 times more. Aerial image of an ordinary, unnamed dam holding a reservoir in green hills, with two drawn bars to scale labelled Medog, China, about 5.5 and SUMP, about 9.2 billion cubic metres.

Power is not protection. Higher generation capacity gives no protection against a sudden release or diversion of water by China. SUMP’s proposed storage is meant to guard against sudden releases. That is why storage is SUMP’s main purpose and power is secondary. These are estimates, and SUMP is still at the PFR (study) stage. An ordinary concrete dam holding back a full reservoir between forested hills.

Why the Siang valley, and not somewhere else?

Locations where storage projects can be planned are limited due to the pattern of flow of the river.

The only solution to mitigate the floods and maintain flow during the lean months in the Siang basin is storage projects at appropriate locations in the valley (immediately downstream of the Tsangpo River).

Projects with flood storage provisions can be planned only in the lower and middle segments of the river, where the slope is moderate.

The land between Pasighat and Guwahati is too flat, which makes it unviable for storage projects. Power generation is incidental.

Has the Siang flooded from upstream before?

Yes. On 11 June 2000, a flash flood came down the Siang. It was caused by the sudden breach of an artificial landslide dam on the Yigong River, a tributary of the Yarlung Tsangpo (Siang), in Tibet.

At Pasighat, the Siang surged to its highest recorded flood level, 167.54 metres. Water levels also rose sharply, by up to 40 metres, upstream at Tuting.

It washed away key bridges and changed the local topography and river course. It claimed 30 lives, left over 100 people missing and displaced more than 50,000 residents in Arunachal Pradesh.

Two glacial lakes in the Siang region hold about 370 million cubic metres of water. If they burst, the flood would be severe, and storage can reduce the danger.

Video · Hindi voice, English subtitles

How can 11,200 MW hold against 60,000 MW?

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