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.

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........

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