Landslide Near Theng Tunnel Partially Obstructs Teesta and Chyakoong Confluence; High Vigilance Maintained Across North Sikkim and Downstream Bengal

KalimNews, Sept 6, 2026, Mangan :  The Sikkim administration has issued a precautionary warning after a major landslide in North Sikkim partially blocked the Teesta and Chyakoong rivers near their confluence, prompting authorities to place vulnerable downstream areas under close watch.

According to the Sikkim State Disaster Management Authority (SSDMA) and the Mangan district administration, the landslide occurred during the intervening night of September 5 and 6 above the Chyakoong River, opposite OC Bhir, in the Naga Forest area near Theng Tunnel. A substantial portion of the hillside along a new road cutting of the Naga–Tosa Road collapsed and fell towards the river gorge.

Exact location of the landslide and river blockade

The danger zone is located in the Naga Forest–Naga-Tosa Road section, opposite the Theng Tunnel, close to the confluence of the Teesta and Chyakoong rivers. The affected slope is therefore not at a distant downstream location; the landslide itself is immediately above the Chyakoong and adjacent to the river confluence.

As a road reference, government records place the Theng Tunnel at approximately km 87.000 to km 87.450 on the Gangtok–Chungthang road. However, the administration's September 6 incident reports identify the landslide by the landmark “opposite Theng Tunnel” and do not provide a separate kilometre reading for the actual debris pile or river blockage. It would therefore be inaccurate to describe km 87.000 or km 87.450 as the exact kilometre of the river blockade.

When the landslide and blockage began

The landslide took place overnight between September 5 and September 6, 2026. The collapse partially obstructed the flow of both the Teesta and Chyakoong rivers, with the Teesta temporarily affected by the debris. Water accumulated in the two river channels, although officials subsequently reported that the Teesta was flowing normally and both rivers continued to flow.

The immediate threat identified by officials is therefore the landslide-generated obstruction at the Teesta–Chyakoong confluence, together with the possibility of further slope movement or a change in river levels if the temporary obstruction changes.

Sikkim administration puts downstream belt on alert

The SSDMA said the situation is being continuously monitored by the District Disaster Management Authority, Mangan, the Police Control Room and personnel deployed in the area. Police personnel at Chungthang and Theng have been conducting ground surveillance.

Authorities have also directed monitoring at the Sankalang and Phidang observation posts. Water levels at both locations were reported to be normal, although the river water had turned muddy. The administration said there was no immediate danger to downstream areas at the time of its assessment, but vulnerable areas in the lower belt were alerted as a precaution.

The administration has advised people to stay away from the Teesta and Chyakoong riverbanks in the affected stretch until further assessment is completed. It has also appealed to the public not to circulate unverified photographs, videos or claims about a possible downstream flood.

No casualties reported

No deaths or injuries have been reported in connection with the landslide so far. Officials are continuing to assess whether the collapse has caused damage to property or infrastructure around the slide zone.

Incident Overview and Geographic Location

• Occurrence Window: The hillside gave way during the overnight hours between September 5 and September 6, following persistent monsoon precipitation across the upper catchment.

• Exact Slide Zone: The collapse originated along an active hill-cutting section of the newly excavated Naga–Tosa Road within the Naga Forest belt, positioned directly opposite the cliff face known as OC Bhir.

• Proximity to Theng Tunnel: The slide occurred directly opposite the Theng Tunnel on the Gangtok–Chungthang highway. While highway markers place the tunnel infrastructure between kilometre 87.000 and 87.450, disaster management officials note that the debris cone and obstruction lie directly within the river gorge rather than along the highway tunnel tube itself.

• Tributary Dynamics: The debris mass descended into the confluence where the Chyakoong River (Chakung Glong Chu) meets the mainstream Teesta. The Chyakoong is a critical 12-kilometre left-bank tributary that originates from high-altitude glacial lakes, including Tosar Lake, traversing international territory across China and India before feeding the Teesta basin.

Hydrographic Conditions and Damming Assessment

• Channel Discharge Status: Heavy boulders, rock scree, and silt choked the narrow riverbed, causing water to pool upstream. However, the damming remained partial. Comprehensive on-site inspections conducted by the Mangan District Administration revealed that the river has naturally carved through the loose debris, allowing a continuous, stable discharge.

• Turbidity and Level Tracking: Downstream observation posts at Sankalang and Phidang confirmed that while the Teesta has turned heavily muddy and turbid, gauge levels remain within normal seasonal margins. Hydrological engineers noted that river monitoring tracks both abnormal spikes and sudden drops, as an unexpected decrease in gauge depth downstream would signify dangerous upstream impoundment.

• Surge Threat: State authorities confirmed that the risk of a sudden flash surge or debris-lake outburst downstream has significantly reduced since morning hours.

Administrative Response and Ground Monitoring

• Sikkim Government Action: Sikkim Chief Secretary Ravindra Telang and the Sikkim State Disaster Management Authority (SSDMA) placed the entire lower river belt on precautionary watch.

• Police and Ground Posts: Continuous surveillance remains active via round-the-clock deployments from the Chungthang Police Station, the Theng Checkpost, and the Mangan District Disaster Management Authority (DDMA).

• Emergency Infrastructure: Control rooms and public helplines were activated across all district emergency operation centres in Mangan, Gangtok, Namchi, Gyalshing, Soreng, and Pakyong.

Downstream Safeguards in North Bengal

Due to the trans-Himalayan drainage of the Teesta into West Bengal, administrative machinery in the plains initiated immediate precautionary protocols:

• Malbazar Subdivision (Jalpaiguri): Civil authorities stepped up vigilance along floodplains. Loudspeaker warnings (miking) were deployed across riverine belts, including Basusuba, Shanti colony and other areas of Gajoldoba, and Bagrakote.

• Precautionary Relocations: Riverbank dwellers residing on vulnerable low-lying sandbars (chars), particularly at Sahebari within the Changmari Gram Panchayat, were shifted to elevated grounds. Similar shelter mobilisations commenced for residents of Totgaon under the Bagrakote Gram Panchayat, with dedicated relief camps readied for occupancy.

• Kalimpong District: While riverine settlements throughout Kalimpong district stay alert to upstream discharge trends, the Kalimpong district administration issued no emergency evacuation directives, classifying the situation as a contained upstream event under active observation.

• Tourism Status: The Confederation of Indian Industry (CII) North Bengal Tourism Panel, led by Chairman Samrat Sanyal, maintained communication with regional administrations, verifying that tourist itineraries remain unaffected and no stranded travelers were reported across travel corridors.

Regional Vulnerability and Weather Outlook

The event generated acute local sensitivity due to lingering memories of the catastrophic October 2023 South Lhonak Lake glacial outburst flood (GLOF) that caused extensive destruction across Chungthang and downstream valleys.

With the India Meteorological Department (IMD) maintaining forecasts for isolated heavy to very heavy rainfall across sub-Himalayan West Bengal and Sikkim, administrative agencies have reiterated their advisory urging the public to stay away from active riverbeds, observe official bulletins, and avoid sharing unverified videos or panic-inducing rumors online. Technical assessments of the unstable hillside above the Theng Tunnel remain ongoing.