When a mountainside gave way before dawn on 24 June, engulfing the NEEPCO residential colony at Poosa village in Arunachal Pradesh’s Keyi Panyor district, the immediate explanation was simple: an extreme monsoon cloudburst had triggered a devastating flash flood.
But nearly two weeks after the disaster claimed five lives, destroyed around 18 residential quarters and severed National Highway-13, earth scientists studying the eastern Himalayas are drawing a far more complex picture. Their assessment suggests that the tragedy was not simply the consequence of extraordinary rainfall, but the culmination of a fragile geological setting, changing climate patterns and decades of human intervention in one of the world’s youngest mountain systems.
“The rainfall was only the trigger,” says Dr Biswajit Bera , geomorphologist at the Department of Geography, Sidho-Kanho-Birsha University, and West Bengal. “The disaster actually began much earlier with the landscape itself.”
A SETTLEMENT BUILT ON UNSTABLE GROUND
At first glance, the NEEPCO colony appears to occupy a broad, level tract of land beside the Poosa stream near its confluence with the Panyor River, known downstream as the Ranganadi. Such flat terrain often appears attractive for settlements in mountainous regions. Geomorphologists, however, see something entirely different . According to Dr Bera, the colony stood on an alluvial fan, a landform created over centuries as mountain streams deposited successive layers of loose sand, gravel, cobbles and boulders after emerging from steep valleys. Unlike bedrock, these sediments remain unconsolidated and inherently unstable. “An alluvial fan is essentially a river’s construction,” he explains. “During periods of extreme rainfall, the river attempts to reoccupy the very channels through which these sediments were originally deposited.” In mountain geomorphology, alluvial fans are recognised worldwide as zones highly susceptible to flash floods, debris flows and channel migration. “People often see flat land and assume it is safe,” Dr Bera says. “But rivers have long memories.”
WHEN RIVERS RECLAIM FORGOTTEN CHANNELS
The NEEPCO colony also occupied another geomorphologically sensitive location close to the junction of two mountain streams. River confluences are naturally dynamic environments where water velocity, sediment transport and flood behaviour change rapidly during heavy rainfall. During the June storm, water from multiple upstream catchments converged almost simultaneously. “The colony effectively became a basin within the drainage system,” Dr Bera says. “Once the water entered, there was little opportunity for rapid drainage.” Geomorphologists often describe such locations as river memory zones areas that may remain dry and apparently stable for years before being reclaimed during exceptional hydrological events. “The river remembers where it once flowed,” he remarks. “Human settlements usually do not.”
THE HIDDEN DISASTER UPSTREAM
While residents experienced the catastrophe as a sudden wall of water, scientists say the destructive process actually began several kilometres upstream. Intense rainfall destabilised steep Himalayan slopes, triggering multiple landslides that temporarily blocked mountain streams. These natural barriers, known as debris dams, impounded large volumes of water behind them. “When these temporary dams fail, they release enormous quantities of water together with mud, rocks and uprooted trees,” Dr Bera explains. “The resulting debris flow is vastly more destructive than ordinary flooding.” Such flows possess tremendous kinetic energy. Massive boulders transported downstream can destroy reinforced buildings, overturn vehicles and strip entire riverbanks within minutes. Residents recall hearing prolonged rumbling before floodwaters arrived. “We thought there had been another landslide somewhere in the hills,” says one survivor. “Within minutes mud entered the colony from every direction. There was simply no time.”
THE RETAINING WALL THAT COULD NOT WITHSTAND NATURE
Many residents believed an under-construction retaining wall near the NEEPCO project would provide protection from seasonal erosion. Instead, it collapsed under the force of the debris flow. A senior Himalayan geotechnical engineer, requesting anonymity, notes that retaining walls are generally designed to stabilise slopes against gradual earth movement rather than resist high-energy debris torrents. “Once large boulders and massive hydraulic pre ssure combine, conventional retaining structures often fail,” experts say. Its collapse allowed an additional surge of sediment-laden water to enter residential quarters already overwhelmed by flooding.
CLIMATE CHANGE IS ALTERING HIMALAYAN RAINFALL
Scientists also place the disaster within a broader climatic context. Across the eastern Himalayas, rainfall patterns have changed noticeably over recent decades. Rather than prolonged moderate rainfall, storms are increasingly characterised by short periods of exceptionally intense precipitation. “The atmosphere is now capable of holding much more moisture because temperatures have increased,” says a climate scientist associated with a national research institute. “The same amount of seasonal rainfall is increasingly falling within a much shorter period.” This concentration of rainfall dramatically increases runoff in steep mountain catchments, where rivers respond within minutes rather than hours. The result is a growing frequency of flash floods, debris flows and slope failures across the Himalayan region.
DEVELOPMENT IN A FRAGILE MOUNTAIN SYSTEM
The eastern Himalayas are among the youngest fold mountains on Earth. Their ge ology remains tectonically active, slopes are naturally unstable, and landslides are part of the mountain-building process. Researchers argue that development has significantly amplified these natural hazards. Road construction, hill cutting, deforestation, quarrying and hydropower infrastructure have altered drainage networks and weakened slopes across large parts of Arunachal Pradesh. “Vegetation provides natural reinforcement to mountain slopes,” a research scholar associated with a national research center says, adding, “When forests are removed, rainfall infiltrates directly into loose sediments, reducing their stability and increasing the likelihood of landslides.” Large engineering projects frequently interrupt natural drainage channels, forcing runoff along new pathways that often intersect with human settlements. “The Himalayas cannot be engineered using conventional plains-based approaches,” he cautions. “Mountain engineering must work with natural geomorphology.”
HYDROPOWER AND HIMALAYAN UNCERTAINTY
The proximity of the colony to the Ranganadi Hydroelectric Project has inevitably prompted questions among local residents. The NEEPCO administration has maintained that power generation was suspended as a precaution and that only controlled releases were undertaken in accordance with operational safety protocols. Scientists emphasise that no evidence has emerged attributing the disaster directly to dam operations. However, they point out that reservoirs inevitably modify local hydrological behaviour, particularly during extreme rainfall events. “Managing reservoirs in steep Himalayan catchments is becoming increasingly challenging as rainfall grows more intense and less predictable,” Dr Bera observes.
RESCUE AMID ISOLATION
The scale of destruction was compounded by the collapse of transport links. According to Shweta Nagarkoti, Deputy Commissioner of Keyi Panyor district, landslides simultaneously blocked access from both Itanagar and Ziro, forcing rescue personnel to reach the colony on foot. Search and rescue operations involving the State Disaster Response Force, police, local volunteers and later the Indian Air Force continued under hazardous conditions as fresh landslides repeatedly threatened rescuers. Chief Minister Pema Khandu, accompanied by Union Ministers Shivraj Singh Chouhan and Kiren Rijiju, later visited the disaster zone, de scribing the devastation as heartbreaking and assuring long-term support for affected families.
THE SCIENCE OF REBUILDING
For researchers, the tragedy raises questions extending well beyond one residential colony. Across the Indian Himalayas, expanding highways, hydropower projects and growing settlements are increasingly occupying river terraces, floodplains and unstable mountain slopes. Hazard zonation maps and geomorphological studies often exist, scientists say, but remain poorly integrated into planning decisions. “Development planning in the Himalayas must b e gin with geomorphology,” Dr Bera argues. “Before approving any settlement or infrastructure project, authorities need to understand how rivers migrate, where floodwaters naturally spread and which slopes are inherently unstable.” The advocate s mandatory geomorphological investigations, high-resolution hazard mapping and continuous monitoring of rainfall and slope stability before large-scale construction in mountain valleys.
A WARNING FROM THE MOUNTAINS
For residents of Poosa village, scientific analysis cannot erase the trauma of that June morning. Many continue searching for missing relatives. Others have lost homes, livelihoods and decades of memories. Yet for the scientific community, the Yazali tragedy represents something larger than a local disaster. It is a reminder that in the Himalayas, landscapes are never static. Rivers continue to re shape valleys, mountains continue to rise and erode, and climate change is making extreme events more frequent. The lesson, researchers say, is not that development should stop, but that it must begin by understanding the mountain itself. Because in the Himalayas, ignoring the language of geology can carry a devastating human cost.
(The writer is a senior journalist at The Statesman)