The Siji River has always followed the same path through Arunachal Pradesh’s Lower Siang district before flowing into Assam. But after a massive landslide crashed into its course, the river came to an abrupt halt.
Instead of flowing downstream, water began collecting behind the pile of rocks, mud, and uprooted trees, creating a large lake almost overnight. While it may appear to be a dramatic natural phenomenon, it has raised concerns for thousands of people living downstream. If the blockage breaks suddenly, the trapped water could rush through the valley, increasing the risk of flash floods in both Arunachal Pradesh and Assam.
The incident has left many wondering how a single landslide could stop an entire river.
Creation of a ‘Landslide Dam’
The landslide occurred near Siji Magi Block Point in Lower Siang district after days of heavy rain. Continuous rainfall had soaked the hillside, making it unstable. Eventually, a large section of the slope collapsed into the Siji River.
With its course blocked by rocks and debris, the river could no longer flow freely. Water from upstream continued to arrive but had nowhere to go, so it began accumulating behind the blockage.
The landslide occurred near Siji Magi Block Point in Lower Siang district after days of heavy rain. Photograph: (The Hindu)
This formed what experts call a ‘landslide dam’, a natural barrier created when debris blocks a river. As more water collects, a lake develops behind it.
Unlike a dam built by engineers, however, this barrier is made of loose earth, rocks, and fallen trees. It was not designed to hold back large volumes of water, which is why such formations are closely monitored.
Although they may sound unusual, landslide dams are not rare in mountain regions such as the Himalayas. Heavy rainfall, earthquakes, and steep terrain make rivers in these landscapes particularly vulnerable to being blocked by sudden slope failures.
Scientists estimate that thousands of landslide dams have formed worldwide over the past century, with many lasting only days or weeks before naturally breaching or gradually eroding.
The eastern Himalayas have seen similar incidents before. In 2000, a massive landslide blocked the Pare Chu River in Tibet, creating a lake that later burst and caused severe flooding downstream in Himachal Pradesh.
In 2018, researchers also documented several landslide-dammed lakes along the Siang river system following seismic activity and landslides, highlighting that such events are a recurring hazard in this region.
Why have authorities issued a warning
The concern is not the lake itself but the possibility of the natural dam failing.
As water levels continue to rise, pressure builds against the debris. If the barrier weakens or collapses, a large volume of water could be released in a short time, sending a powerful surge downstream.
As water levels continue to rise, pressure builds against the debris. Photograph: (The Hindu)
The Siji River eventually joins river systems that flow into Assam, prompting authorities in both states to issue alerts. Residents have been advised to avoid riverbanks and stay prepared for any change in water levels.
Authorities have not yet released official estimates of the length of the blocked river, the size of the newly formed lake or the volume of water trapped behind the debris.
However, officials say the impounded water is major enough to warrant continuous monitoring and evacuation alerts for settlements downstream, including around Likabali and neighbouring areas extending into Assam.
The incident comes amid an active monsoon across the eastern Himalayas, where prolonged rainfall has triggered landslides, swollen rivers, and flooding in several districts. Heavy rain weakens mountain slopes by saturating the soil, making landslides more likely.
The landslide did not cause the floods
While the blocked river has become the focus of fresh warnings, it is not responsible for the floods that have already affected parts of Arunachal Pradesh.
The flooding began after several days of intense monsoon rain, which caused rivers to swell, mountain streams to overflow and triggered landslides across multiple districts. The same spell of heavy rainfall also destabilised the hillside above the Siji River, eventually leading to the collapse that blocked its course.
As water levels continue to rise, pressure builds against the debris. Photograph: (The Financial Express)
The landslide has instead created a new risk.
By damming the river, it has trapped a large volume of water upstream. If the natural barrier gives way suddenly, that water could surge downstream, increasing the risk of flash flooding in parts of Arunachal Pradesh and Assam. This is why authorities have issued alerts for communities living along the river, even as they continue to deal with the wider impacts of the ongoing monsoon.
In short, the heavy rain caused both the floods and the landslide. The blocked river has not triggered the current flooding, but it could worsen the situation if the temporary natural dam collapses.
In such situations, authorities usually avoid disturbing the natural barrier immediately unless it shows signs of imminent failure. Instead, they closely monitor water levels, rainfall and the stability of the debris using field inspections, satellite imagery and drones where possible.
If the risk increases, evacuations are prioritised, and engineers may create a controlled channel or spillway to gradually release water and reduce pressure on the natural dam. These measures are recommended in disaster management guidelines for landslide-prone mountain regions.
For now, officials are monitoring the natural dam and the lake that has formed behind it. Whether the water drains gradually or the blockage gives way will determine how the situation unfolds over the coming days. Until then, the focus remains on keeping downstream communities safe.
