Landslides that struck Nagaland’s Longleng and Mon districts on July 19 have revived fears that parts of the Dikhow basin could face prolonged ground instability.
Residents of Tamlu and nearby areas say cracked homes, damaged roads, vanished streams and altered watercourses have left them anxious about whether the land is still moving.
The disaster has drawn comparisons with Noklak and Tuensang, where landslides and land subsidence have expanded over years, though geologists say more investigation is needed before making direct links.
Experts say heavy rainfall was the most likely trigger, but road construction, slope cutting, drainage conditions, geology and other human activities need closer scientific scrutiny.
The monsoon disasters that battered South and East Asia this year also left a deep mark on India’s northeast.
In Assam’s Sivasagar, Charaideo and Jorhat districts, people living on the floodplains witnessed sudden flash floods overlapping with the region’s usual riverine flooding. Both originated in the same river system — the Dikhow basin, known as Dikhu in Nagaland.
Upstream in Nagaland, however, the disaster unfolded differently. Landslides struck parts of Longleng and Mon districts on an unusual scale on July 19, leaving residents fearful that some areas could be entering a longer cycle of land subsidence.
On the morning of July 19, residents in parts of Longleng and Mon woke to cracking sounds in the walls of their houses as the ground beneath them began to shift.
In some places, entire slopes collapsed, taking houses with them. Elsewhere, homes were left hanging precariously as residents were evacuated. Roads were damaged or cut off, isolating towns and villages.
Down To Earth reached Tamlu on August 8 via the Amguri-Assam-Tuli-Nagaland route, then the only route from Assam to the town. After Tuli, the scale of destruction became clear. Landslides had scarred the road at several points, from small slips to large slope failures. In some stretches, the road had narrowed so sharply that only one vehicle could pass at a time.
Wanthah, a 43-year-old resident of Tamlu, said this was the first time he had seen such a disaster in the town. Pointing to cracks in houses and roads, he said Nyeichong ward had suffered the worst damage. Residents described a clear pattern: a large section of the slope around Tamlu appeared to be slowly moving downwards.
Travelling directly from Tamlu to Mon was impossible, forcing people to return to Assam and take the Sonari-Tizit-Mon road. After that difficult journey, Mon appeared to be a town surrounded by landslide-hit slopes.
Yoam Konyak, action secretary of the Konyak Students’ Union, recalled the morning of July 19, when an entire slope below an Assam Rifles camp collapsed, taking 10 to 12 houses with it.
“Early morning that day, people woke up to huge cracking sounds and people screaming,” he told DTE on August 9. “Without any warning, this happened at once. Eight people were buried alive. We could recover six bodies, but two simply vanished beneath the ground. Many houses developed cracks, tilted to one side, and many were left hanging after the ground beneath them moved away.”
Landslides are common in hilly terrain, but residents and officials said what happened in Mon and Longleng was unusual because of its scale and simultaneity.
DTE spoke to Longleng Deputy Commissioner W Manpai Phom, who said every village in the district had suffered damage.
“Although Tamlu witnessed the maximum damage, the Yongnyah area, situated on the opposite bank of the Dikhu River, was no different in terms of the scale of losses. For four to five days, many villages remained isolated as inter- and intra-district roads collapsed or were blocked,” he said.
The Dikhu flows through Longleng district, with Tamlu on its left bank and Yongnyah on the right when approached from Assam through Amguri and Tuli. Phom said valley areas downstream of Tamlu also faced flash floods. Newly planted paddy saplings in the Namsang area were washed away, he said.
The same area was hit again on August 29. Wanthah said a channel known as Shenney caused the July 19 flood in paddy fields downstream of Tamlu. Farmers replanted paddy, only for the crop to be washed away again by the same channel. “This is not the usual course of Shenney. It changed on July 19, and again it took the changed path on August 29,” he said.
Phom said some waterfalls or streams had vanished upstream, while new ponds, which he estimated to be about 50 feet deep, had formed on both the Tamlu and Yongnyah sides.
Tingak Konyak, a 54-year-old teacher near Mon who is from Tamlu, said he had never heard of anything like this in Tamlu, Longleng or Mon. “What makes it unusual is that the landslides are not confined to some particular landslide-prone areas. Moreover, the landslides occurred within the same time window everywhere,” he said.
Two rivers flow through Mon — the Tapi to the east and the Tenang to the west — before discharging into the Dikhu. Yoam Konyak said more than 200 houses were affected in Mon town alone, with 70 completely destroyed.
He said seven to eight large landslides and many smaller ones hit Mon town, while other parts of the district also suffered damage. About 1,110 students were affected after losing uniforms, books, documents or homes, he said. Five schools were damaged, including one in Mon town and four in Tizit. The government middle school in Mon suffered major damage and awaits repairs before its students can return to classes.
Floods also caused damage in Naginimora and Tizit, the other headquarters of Mon district.
The destruction has revived fears of the slow-moving crises already seen in Noklak and Tuensang.
A few days before the July 19 landslides, the District Transport Office in Tuensang collapsed after a landslide. There were also reports of roads and stretches of national highway in Noklak being washed away. For residents, these are not isolated landslide events. In Noklak, what began as a minor landslide in 1980 had, by 2022, subsumed nearly a quarter of the town, and the ground is still sinking.
Thanghiu of the District Disaster Management Authority, Noklak, told DTE that nearly 10 inches of the town had sunk so far this year. The landslide, which began in the southwestern part of Noklak, has spread over time. This year, landslides were reported in Punyaongan, towards the east, he said. A section of the upper part of the town’s main road has disappeared. Where houses once stood, the ground has now sunk below road level, leaving the houses lower than the roadway.
Independent researcher Tosang Chang recalled a landslide in Noklak from her childhood: “One morning, I woke up to find a large gathering at our house. My father said we were moving to grandma’s place. The stairs to a stream behind our house had disappeared as the land beneath them moved down, while parts of the house had cracked and tilted.”
Chang said people still live in cracked houses in Noklak because not everyone can afford to move. “The administration asks them to vacate the houses, but only the well-to-do people can afford it easily,” she said.
Tuensang shows a similar pattern. What began as a landslide in Phinjang colony in the late 1990s has expanded over time. Moses Hongang Chang, a local journalist with the newspaper Morung Express, said the crack line from Phinjang had spread to different locations, including Tuensang village 2.5 kilometres away. A newly built church 4 km away developed cracks, while another area 5 km away has also been affected.
He said stone quarrying from a stream below Phinjang, a tributary of the Dikhu, may have contributed to instability. The administration stopped quarrying only last month after public concern grew, he said.
What Noklak and Tuensang have in common is that a massive landslide, initiated in one place, expands over time and becomes a recurring event. In a 2021 study, geologists from Nagaland University identified the geological factors contributing to Noklak’s sinking, which have been further aggravated by anthropogenic activities.
The pattern is fuelling anxiety in Tamlu and Longleng. “Will we end up meeting the same fate as people of Noklak or Tuensang?” is the question residents are now asking.
The fears were further fuelled by what people were seeing. Tingak said, “Just two weeks ago, while travelling from Mon towards Wanching in Longleng, I saw land sinking in the plains between Wakching and Tanhai. There is a tea garden there.” Others, including Yoam Konyak, also said they had seen land sinking in the Wakching area.
Temsulemba Walling, a senior geologist and professor at Nagaland University, said more investigation was needed before Tamlu could be compared with Noklak.
He said the Mon and Tamlu highway landslides were triggered by heavy precipitation on July 19 and occurred on steep slopes with unfavourable geology and probably poor drainage. Noklak, by contrast, is a complex landslide in the Disang Group of shales, probably also influenced by neotectonic movements. “While the Tamlu slide happened in the Disang shales, the Mon landslide occurred in the Laisong Formation of the Barail Group,” he said.
Walling said the Noklak landslide involves toe erosion (wearing away of soil or rock at the base of a slope), debris movement, subsidence and tensional fractures across parts of the town. The two drainage systems, Khiamong and Lein rivers, are eroding debris, but the scale and persistence of movement suggest a deeper and more complex instability, possibly linked to active tectonism.
There were wet spells in the preceding week and heavy rain on July 18-19, but this does not qualify as a cloudburst. Sarat Phukan, a geologist at Gauhati University, said rainfall was the most likely trigger in Tamlu.
He said the upper Dikhow catchment received about 123.1 millimetres of rain between July 12 and 18, and 117.2 mm between July 18 and 19, according to NASA’s Integrated Multi-satellitE Retrievals for GPM data. Tamlu received about 165 mm and 94.2 mm over the same periods.
“The most likely explanation is rainfall-triggered local or compound shallow-slope deformation involving soil, colluvium, road fill and weathered shale,” he said.
Rain during the preceding week would have raised soil moisture and shallow groundwater levels, while heavy rain on July 18 and 19 increased pore-water pressure and weakened slope material, he said.
Phukan added that further movement at the same site was possible during future periods of prolonged or heavy rainfall, including future monsoons. But current information was insufficient to estimate the probability, depth or likely size of such movement.
Y Nuklu Phom, winner of the Whitley Award in 2021, said the Longleng disaster may have involved several factors: heavy rainfall, climate change, possible tectonic activity and human interventions. He said rainfall increased by 57 per cent in July, raising the water volume of the Dikhu and affecting the basin system. But he also pointed to road construction and landslides along the Dikhu and Yangmon river belts.
“For the past 10 to 12 years, the National Highways and Infrastructure Development Corporation Limited has been carrying out heavy road construction projects in the region, expanding to Mokokchung from Longleng,” he said. “I feel that hill cutting, deforestation and the use of heavy dynamite for blasting rocks have significantly contributed to the disasters, as these processes have weakened the slopes.”
“Sitting in my office, I could hear the sound of dynamite blasts 7-8 km away. The National Highway 702, which connects Longleng, runs along the Dikhu River, as does the Yongnyah-Tamlu road, which crosses the river. Major landslides occurred along the Dikhu and Yangmon river belts,” Nuklu added.
Nuklu also suggested that long-term use of salt and other chemicals by farmers as weedicides may have affected soil character, though this remains contested.
On the possibility of seismic activity, Phukan said the National Centre for Seismology catalogue recorded no nearby earthquake at the time of the July 19-20 ground movement. Earthquakes recorded on July 17 and 18 were too small, distant and deep to explain the observed damage, he said. “While a very small local earthquake cannot be completely ruled out due to the lack of nearby seismic stations, there is currently no evidence that an earthquake caused the ground movement in Tamlu,” he said.
Coal mining in the Bura Namsang area along the Dikhu has also been raised as a possible contributor. Nuklu said mining had increased in recent years and could have added to siltation and bank erosion.
Walling said there was not enough evidence to directly link coal mining to disruption of the Dikhu. Mining in Nagaland’s Schuppen Belt has existed for decades, he said, but its impact was more likely to be limited to smaller streams and tributaries. Any link to the recent events would require investigation using multi-temporal satellite data. “Available data also indicates a very low correlation between coal mining and landslides in Nagaland,” he said.
Whether Tamlu is witnessing the early stages of a prolonged geological process like Noklak or Tuensang remains unanswered. For now, residents are left watching cracks in the land and waiting for authorities to investigate before the next monsoon turns today’s warning signs into a larger disaster.