Severe Flooding in Sivasagar: The Impact of Environmental Degradation
Historic Flooding in Sivasagar
The old Dikhow bridge in Sivasagar town.
Sivasagar, Aug 5: Four years after the Dikhow river first raised alarm by turning acidic and causing large-scale fish deaths, the same river has unleashed one of the worst flood disasters in Sivasagar’s history. Since July 19, the river’s unprecedented surge has claimed 87 lives, displaced thousands, and renewed scrutiny over the cumulative impact of mining, deforestation and large-scale earth cutting in its upstream catchment.
Experts believe the catastrophe may not have been caused by extreme rainfall alone. Satellite imagery has revealed extensive excavation, deforestation and other human interventions along the river’s upper reaches, particularly in Nagaland, raising concerns that decades of environmental degradation may have intensified the disaster.
Coal mining, stone quarrying and shifting (jhum) cultivation have stripped large stretches of hill slopes of vegetation, making them increasingly unstable and vulnerable to erosion and landslides.
“The speed at which the river rose last month indicates that factors other than heavy rainfall may have been at play,” said Dhrubajyoti Sahariah, researcher and head of the Centre for Brahmaputra Studies at Gauhati University.
“It resembled a glacial lake outburst in terms of the sudden surge. One possibility is that the river was temporarily blocked by landslides or debris. When the obstruction gave way, an enormous volume of water rushed downstream within a very short period,” he said.
The river’s behaviour during the recent flood was unprecedented. Within just a couple of hours on July 19, the Dikhow crossed the warning level, breached the danger mark and climbed to its highest recorded flood level, leaving little time for residents to respond.
In non-glacial regions, Sahariah said, the closest equivalent to a Glacial Lake Outburst Flood (GLOF) is a cloudburst-induced flash flood or a dam/landslide outburst flood. These events share the same sudden, destructive nature as GLOFs but are triggered by intense rainfall or failure of natural/artificial barriers rather than melting glaciers.
However, the IMD has claimed there was no cloudburst, and officials have denied the existence of any dam on the river upstream.
Sahariah also pointed to unusually high levels of silt and sediment deposited along the Dikhow’s banks.
“A large amount of mud and debris is being washed down from the hills, reducing the river’s carrying capacity and increasing its vulnerability to flooding,” he added.
Geologists say the hill ranges of Nagaland are naturally prone to slope failures. The region is underlain by the fragile Disang rock formations, which possess poor geotechnical strength and are highly susceptible to weathering, landslides and slope instability, especially during periods of intense monsoon rainfall.
A report by the North Eastern Regional Disaster Management and Research Centre (NERDDR), prepared following the recent landslides in Mon district, noted that while extreme rainfall was the immediate trigger, pre-existing slope deformation and progressive ground movement within the tectonically active Naga Schuppen Belt could also have contributed.
The report further observed that changes in land use, including loss of vegetation, altered surface drainage and modification of hill slopes, may have significantly reduced slope stability.
Mon district, which borders Sivasagar district, witnessed more than 60 landslides following the heavy rainfall on July 19, according to the district administration. The banks of the Dikhow in the district are dotted with coal mines and stone quarries.
The disaster has also revived concerns first flagged in 2022, when the waters of the Dikhow turned bluish and widespread fish deaths were reported.
Subsequent laboratory tests conducted by the Pollution Control Board, Assam (PCBA), found the river’s pH to be in the acidic range. At the time, the board had observed: “There may be some activities in the upstream of the river which have decreased the pH value of the river water.”
Originating in Nagaland’s Zunheboto district, the Dikhow enters the Assam plains at Naginimora before flowing through the Sivasagar district and joining the Brahmaputra at Dikhowmukh. The river stretches over 236 km, draining a catchment area of 4,372 sq km, of which about 98.5 km lies within Assam.
Over the years, the river has left behind more than 15 abandoned channels across the Assam plains, many of which now exist as oxbow lakes.
According to an earlier PCBA report, a 6-km stretch of the Dikhow between Hatimuria and Arjunguri, covering approximately 17.6 sq km, has been identified as polluted. The report had also warned that increasing encroachment along the riverbanks was narrowing and shallowing the channel, further diminishing its capacity to safely carry floodwaters.