Chunks of the hill crashed about 300 metres from the dam.
A massive landslide hit an under-construction dam in Arunachal Pradesh on Friday, affecting a 2,000 megawatt (MW) hydroelectric project and reducing water flow in Assam's Subansiri river.
Chunks of the hill came crashing down about 300 metres from the dam, showed a video that has gone viral, blocking a diversion tunnel in the Subansiri river and leading to a drastic reduction in the downstream waterflow.
This was the only diversion tunnel in use at the Subansiri Lower Hydroelectric Project as the other four diversion tunnels had already been blocked earlier, the National Hydroelectric Power Corporation (NHPC) said.
This comes weeks after another disaster hit the Northeast - a dam breach in Sikkim following a glacial lake outburst caused massive flash floods and several deaths.
The landslide has alarmed the authorities in Assam's Lakhimpur district, downstream of the dam in Arunachal Pradesh.
The government has issued an advisory, asking people to restrain from activities like fishing, swimming, bathing and boating. People have also been asked to keep their cattle away from the river.
The NHPC said the river is expected to return to its usual flow by the evening. "The current river flow of 997 cubic metres per second is being stored in the reservoir, and the water level reached 139 metres by 1 pm. It is expected that the water level will reach 145 metres by the evening, and the river will return to flowing through the spillway normally," the statement read.
A landslide had earlier blocked the four diversion tunnels and they are not currently in use. These diversion tunnels are used to redirect river water during construction.
In the past decade, the construction of the dam had faced opposition from activists and locals who feared a downstream impact.
In April last year, the powerhouse protection wall had collapsed due to tail race channel construction activities. The tail race channels, which release water back into the river after passing through turbines, are located on the right bank of the river in Arunachal Pradesh.
Over the past three years, the project site has been hit by four major landslides. The NHPC began construction of the project in January 2005, but work was stalled between 2011 and 2019 due to protests.
According to a company estimate in 2020, the cost of the project, originally scheduled to have been commissioned in 2012, had escalated to around Rs 20,000 crore from the initial Rs 6,285 crore.