On June 24, the Poosa area of Arunachal Pradesh’s Keyi Panyor district woke up to horror. A huge gush of water washed away houses of the NEEPCO (North Eastern Electric Power Corporation ltd.) colony and nearby Poosa village under the Yazali circle.
The NEEPCO colony is the residential complex for officials of the Panyor Lower Hydroelectric project, known as the Ranganadi dam in local parlance. News came flashing about the horrific incident with speculation about cloud bursts as well as release of heavy water through the dam gates.
Dams or hydroelectric power projects have been an issue of serious concern in Northeast India, with wide scale resistance against them. Massive protests last year against a proposed dam over the Siang river is still fresh in memories of the people of Arunachal Pradesh. A devastation of the scale that occurred at Poosa readily triggered speculation that a cloud burst in the upper regions of the state had accumulated heavy water in the dam and that its gates were opened, causing the devastation or that the dam had broken.
Notably, the Ranganadi dam has been a concerning issue for Assam, especially for its impact in Lakhimpur district. The dam is over the Ranganadi river that originates in Arunchal and flows down to Lakhimpur district of Assam before meeting the Brahmaputra.
Pill Reka, a youth of Yazali village, 9 kilometres away from the devastation site told Down To Earth (DTE) that there was heavy rain throughout the night of June 23 that continued till noon next day. He thinks that the incident to be a cloud burst.
However, according to the India Meteorological Department (IMD), a cloud burst is defined as 100 mm of rainfall in an hour. A Sandeep, the head of the meteorology center, Itanagar said there was 73 mm of rainfall in 24 hours prior to the flash flood, from 8.30 AM of June 23 till 8.30 AM of June 24. “However, most of the rainfall occurred during 5.30 AM to 8.30 AM of June 24,” he said confirming that it doesn’t comply with IMD’s definition of a cloudburst.
Reka also denied that the flash flood was due to opening of the Ranganadi dam gates as the NEEPCO colony is situated upstream of the dam gates. “The heavy water came through a channel known as ‘Poosa river’ locally. The dam is on the Panyor river (or Ranganadi river),” he said.
An official of the SDMA (State Disaster Management Authority) apprised DTE that the water released from the dam was around 10 AM. “The discharge through the gate was 201.413 cumec while the inflow was 163.36 cumec,” he said.
Mirza Zulfiqur Rahman, an independent researcher on the Brahmaputra basin however thinks that hydropower development has a contributory role in the devastation. “Any structural intervention in the riverbed, such as a dam, causes flow changes both upstream and downstream of the dam site. The accumulated rain water would have dissipated in a different manner and the devastation would be much lesser. The dam head always contains substantial amount of water stored for electricity generation, which can substantially add as a backflow to the accumulated flows caused by the rainfall, adding to the severity of the disaster upstream of the dam site. Often it is assumed that only the downstream areas are worse affected due to a dam, but upstream areas can also suffer serious consequences through altering the natural drainage conditions,” Mirza told DTE. “There could be a change in the micro climate of the catchment area that can shift the region of cloudburst or severe downpour, which is known to occur in the lower catchment area of the Ranganadi dam site. We don’t have a proper scientific study on it,” he added.
Sarat Phukan, a geologist and senior professor at Gauhati University, offered a different interpretation of the recent flash flood. “Backwater effect from the reservoir can cause inundation in low-lying areas. However, based on the reported force and velocity of the floodwaters, the devastation at the NEEPCO colony is unlikely to have been caused by reservoir backflow alone. Backwater may raise the water level, but it does not usually generate a high-energy flash flood,” Phukan said.
“At this stage, without a detailed geomorphological and hydrological investigation, at least two plausible mechanisms deserve consideration. First, the small mountain channel flowing through the NEEPCO colony may have been temporarily blocked upstream by landslide debris, sediment, or fallen trees. Intense rainfall could have impounded water behind such a natural blockage, and its sudden breach would have released a powerful flood wave downstream. Second, a highly localised cloudburst may have occurred farther upstream in the catchment, beyond the coverage of available rain gauges. In steep Himalayan terrain, such events can produce destructive flash floods within a very short time.”
A Sandeep told DTE that the 73 mm rainfall data was recorded by the station at Yazali, a distant node of the possible remote areas that Phukan suggests.
Drawing a geological analogy, Phukan added, “Following the great Assam earthquake of 1950, several rivers and streams in the eastern Himalaya were temporarily dammed by landslides. The Subansiri river, for example, remained blocked for several days in its upper reaches, and when the natural dam eventually failed, it caused widespread devastation downstream in Assam. Although the present event is much smaller in scale, the mechanism of temporary blockage followed by sudden release is a well-recognised geomorphic process worthy of investigation.”
Keshab Krishna Chatradhara, a prominent river activist based in Assam also says that the opening of dam gates cannot be attributed as being behind the incident. Chatradhara also thinks that the small channel was blocked in the upper reaches, which later opened due to heavy rain. “As the devastation site is upstream of the dam elevation, it is not likely that the dam has caused the devastation of that scale. Moreover, had the dam got broken it would have caused severe problems in Assam.”
The incident gave no time to anyone. Early in the morning (between 6.30 and 8.30 AM), a huge wall of water came rushing down with a massive sound and within moments, the entire area became unreachable. Pill Moni, a young faculty at the Deranatung Government College in Itanagar, hails from Yazali village. She got worried for her relatives at the devastation site. Moni told DTE that she was unable to contact her relatives and with great difficulty got to know that they were safe only late in the afternoon.
The SDMA told DTE that till June 25, one death and four disappearances have been reported with several people injured. But the exact number of people affected and total loss is still being assessed. The residents of the NEEPCO colony and the villages have been evacuated to safer places by SDRF, NDRF and local authorities. Beru Dulom, the district disaster management officer of Keyi Panyor district, told DTE that about 18 residential quarters of the NEEPCO colony were washed away by the furious gushes of the flash flood.
According to a local journalist Manoj Singh, around 50 houses were washed away across the NEEPCO colony and the Poosa area along with several cars and livestock. He added that lanslides in about 50 places between state capital Itanagar and Pitapool (near the devastation site) have blocked road communication. “I was awaiting to go to Poosa in helicopter from Naharlagun along with others, but the copter didn’t take off due to the weather conditions. The critically injured patients were taken to Ziro as all roads to Itanagar were blocked,” he said.
An officer at SDMA, on condition of anonymity, gave a short account of previous such incidences. “These kinds of events have been happening regularly here. It has been almost a decade now since I am posted here and I have seen many such occurrences. In 2025, a similar incident occurred at Dirang. In 2023 or 2024, there were severe damages at Darjeeling village in Mechuka. In 2024, a local stream in Itanagar washed away many houses.” Notably, all these incidents occurred without specific warnings, he pointed.
When contacted, a staff member at the State Emergency Operation Centre told DTE that they received a yellow alert on June 23 for the Lower Subansiri district (Keyi Panor is a newly crafted district with parts from Lower Subansiri district). However, on June 24, the meteorology brought an orange alert after the devastation.
The meteorology centre produces district level warnings for the next five days and forecasts for the next seven days. Although the prediction models the IMD uses can produce block level forecasts, they restrict it to the district level for operations.