What is the reason behind the devastating floods that swept across Upper Assam, claiming more than 80 lives and displacing over seven lakh people of 800 villages across 11 districts? Initially, the government attributed the floods to extremely heavy rainfall in the Naga Hills caused by a cloudburst. However, the India Meteorological Department (IMD) said there was no cloudburst in the Naga Hills.
There were also allegations that excess water had been released from the reservoir of a smaller project, the Dikhu Hydroelectric Project (DHEP). DHEP has three units of 62 MW each, with a total installed capacity of 186 MW. None of the three units — I, II or III — of the Dikhu Hydroelectric Project are operational; the project is still tied up in land disputes and has not reached the construction stage, let alone commissioning. Construction of the dam and reservoir has not been completed, so the question of releasing excess water does not arise.
What triggered flooding on such an unprecedented scale? How did the Dikhu river overflow into the homes of more than seven lakh people, inundating more than 800 villages, many of which had not experienced flooding in the past 70 years?
The more important story begins upstream in the Naga Hills, where more than a century of coal mining, widespread hill cutting, deforestation and riverbed stone extraction have steadily altered the way rivers respond to rain. Over the years, these activities have reduced the catchments’ ability to absorb rainfall and slowed the rivers’ capacity to carry floodwaters. The July floods exposed the cumulative impact of those changes.
The Dikhow River, known as the Dikhu in Nagaland, rises near Nuroto Hill on the Zunheboto-Kiphire divide. It flows through Zunheboto, Mokokchung, Longleng and Mon districts before entering the Assam plains at Naginimora and eventually joining the Brahmaputra at Dikhowmukh.
The river basin is home to Nagaland’s only major coalfield, centred in the Naginimora subdivision of Mon district around Borjan and Kongan-Tiru. Coal has been extracted from this hill continuously since 1907, with mining activity concentrated along the river and the adjoining hill slopes.
Two coal mining systems continue to operate in the belt. Borjan is dominated by underground rat-hole mining, while Upper Tiru uses the open-cast method, in which excavated overburden is dumped onto nearby slopes rather than being scientifically contained or restored.
Over the decades, Naginimora has emerged as Nagaland’s principal coal trading centre. Local accounts indicate that nearly 300 trucks carrying coal and river stone pass through the town every day.
The trade in river stone is as significant as the coal trade. It reflects the large-scale extraction of boulders and gravel directly from the Dikhow riverbed, separate from mining activity in the surrounding hills. Removing stone from a river channel weakens both the riverbed and its banks, making them more vulnerable to erosion and adding to the pressure created by sediment flowing down from the hills.
Much of this activity has continued with limited regulatory oversight. Nagaland’s coal sector is governed by the Nagaland Coal Policy, 2006, which permits small-pocket rat-hole mining by individual landowners instead of regulating mining on a larger scale. Article 371A of the Constitution, which gives local communities ownership over land and its resources, further limits the extent to which state or central agencies can intervene.
The National Green Tribunal (NGT)’s 2014 ban on rat-hole mining, imposed after the mining tragedy in Meghalaya, has had little practical impact on Nagaland’s small-pocket mining regime. By most accounts, enforcement across the coal belt remains weak.
Research published over the years has linked both rat-hole and open-cast mining in Mon and Wokha districts to widespread environmental damage, including deforestation, unstable hill slopes, soil erosion, acid mine drainage and deterioration in water quality. These impacts are not confined to mining sites. They extend downstream through the river system.
Mining is only one part of the problem. Geographers, including Professor Abani Kumar Bhagabati of Gauhati University, have long pointed to deforestation and unplanned hill cutting as major factors behind the increasing severity of floods in Assam.
Forests play a crucial role in regulating runoff. Tree roots bind the soil, stabilise slopes and allow rainwater to seep gradually into the ground. Once the forest cover is removed, rainwater runs off the surface much more quickly. Soil erosion intensifies, landslides become more frequent and a greater share of rainfall flows directly into streams instead of being absorbed.
Hydrologists describe this as a reduction in the catchment’s lag time — the gap between rainfall in the hills and floodwaters reaching the plains. As that interval becomes shorter, rivers rise faster after heavy rain, leaving downstream areas with less time to respond.
Road construction has added to the problem. Large stretches of roads have been built or widened across the Naga Hills, including foothill routes and roads developed to transport coal from the Naginimora belt. In many places, debris from cut hill slopes is dumped along the roadside or pushed directly into small streams at the headwaters.
This sediment enters the river system almost immediately, increasing the volume of material carried downstream long before the rivers reach the Assam plains.
Sediment washed down from the hills eventually settles where rivers lose speed and gradient, particularly after they enter the embanked stretches in Assam. Ironically, the embankments built to protect flood-prone areas also trap much of this sediment within the river channel by preventing it from spreading across the floodplain during high flows.
Over time, the riverbed rises between the embankments, leaving some channels flowing above the surrounding countryside. Upper Assam’s tributaries were already carrying large quantities of silt deposited after the 1950 Assam earthquake. Decades of mining, hill cutting and erosion have added another layer of sediment to an already stressed river system.
The result has been a gradual decline in the rivers’ ability to carry water. As channels become shallower and narrower because of sediment deposition, flows that remained within the banks decades ago are now more likely to spill over.
Hydrologically, this is reflected in rivers reaching danger level and crossing the Highest Flood Level (HFL) after comparatively lower rainfall than in the past. That was the pattern seen during the July floods, when rainfall within the region’s historical range was enough to push several rivers beyond their carrying capacity.
The impact is not confined to the hills. It continues downstream after these rivers enter the Assam plains.
The Jhanji and the Bhogdoi, known as the Tsurang in Nagaland, originate in the Mokokchung hills and flow towards Jorhat, entering the plains near Mariani. Like the Dikhu, both rivers pass through stretches where boulders and gravel are extracted directly from the riverbed without effective regulation.
During the July floods, an embankment breach on the Jhanji inundated the Jamuguri area, while another breach on the Dorika flooded tea estates across Nazira subdivision.
Those failures were part of a larger pattern. Jorhat district had already recorded six embankment breaches since July 17. Even before the main flood wave reached Upper Assam, the state had reported 18 embankment breaches and damage to 25 other flood-control structures between April 29 and July 6.
The flood protection network had already come under strain before the July deluge. At the same time, the rivers were carrying increasing quantities of sediment from upstream catchments where forests had been cleared and coal and stone extraction had continued for decades.
The geography of these rivers leaves little room for error. The Disang, known as the Tisa in the hills, drops from about 2,200 metres to around 100 metres over a distance of nearly 65 km before entering the plains near the Sonari-Naginimora belt and joining the Brahmaputra at Disangmukh.
Unlike many Himalayan rivers, these hill rivers have almost no broad foothill zone where floodwaters can slow before reaching inhabited areas. Hydrologists refer to this as the absence of a piedmont buffer. As a result, rain falling on the ridges of Nagaland can reach the Sivasagar plains within hours rather than days.
When that rapid runoff is combined with deforested slopes that absorb less water and river channels whose carrying capacity has steadily declined because of silt deposition, flooding becomes both quicker and more severe.
By the middle of July, soils across the hill catchments were already saturated after days of rain. Any additional rainfall flowed almost entirely over the surface instead of soaking into the ground. Hydrologists describe this as a “saturation-excess” flood, where even rainfall within the normal range can trigger major flooding because the land has already lost its ability to absorb more water.
In such conditions, degraded catchments and sediment-choked rivers were enough to produce widespread flooding without requiring an extraordinary rainfall event.
The July floods therefore point to a much deeper problem than a single spell of heavy rain. More than a century of coal mining with limited regulatory oversight, continued extraction of stone from the Dikhu riverbed, hill cutting for coal transport roads and widespread deforestation have gradually reduced the catchments’ capacity to retain water and the rivers’ ability to carry it safely downstream. None of these processes is new. Each has been documented for years. What the July floods showed was the cumulative effect of these long-term changes, with rivers reaching danger level and crossing previous flood marks at rainfall levels that might once have remained within their banks.
Former Congress MLA Debabrata Saikia also blamed illegal sand mining in the Dikhow riverbed for worsening the floods.
“The flood is only due to illegal sand mining on the Dikhow riverbed. If you go to Bihubar, you will see how the river has been torn apart,” he said.
Saikia alleged that floodwaters damaged houses and infrastructure in Nepali Khuti, Bihubar, Ramani, Rahdoi, Simaluguri, Napukhuri, Dulakshoria and more than 500 other villages.
He also claimed that on the night of July 19, heavy rainfall turned the road connecting Mon town in Nagaland into a river, while landslide debris blocked the Dikhow river.
Professor Arup Kumar Sarma, hydrologist and Professor of Civil Engineering at the Indian Institute of Technology (IIT) Guwahati, said developmental activities in hilly areas can significantly alter flood behaviour downstream if they are undertaken without proper hydrological assessment or ecological safeguards.
“Developmental activities like building roads or other infrastructures cutting down a lot of trees for mining or other work in hilly areas if done without a full study of how water moves and without taking care to grow more plants can change the flood situation a lot downstream especially when there is heavy rain in the hills that causes a lot of water and dirt to flow. Not having space for streams to pass through roads can cause flooding and landslides that stop communication. Not having space in roads, in flat areas can block sudden floods causing water to stay in one place and flood areas upstream of the roads.”
Sarma said two factors were primarily responsible for the worsening flood situation.
“There are two reasons. Extensive deforestation in the upper catchment is responsible. For example extensive deforestation in the upstream hills of Nagaland (for Sibsagar and Jorhat); Arunachal Pradesh (for Dhemaji and Dibrugarh); and Meghalaya (for Khanapara, Jorabat and Guwahati) is seen in the google map. Fig-1 is given as an example of Dikhow River. Such deforestation due to boulder mining, road construction and other developmental activities. Such deforestation can generate water and sediment yield. Sediment deposition makes the river shallow. Thus they loss their water carrying capacity.
“We are experiencing high intensity rainfall due to climate change. Extreme event in Eastern Himalaya due to impact of El-Nino this year cannot be nullified, in some part El-Nino causes low precipitation though. Cyclonic moisture movement from bay of Bengal towards Assam moving over Nagaland and Manipur is seen, which can cause precipitation due to orographic lift. Continuous precipitation causes land saturation and a high intensity precipitation over saturated deforested land generates high water yield.”
Sarma added: “If such precipitation happen plain area of these districts can also experience flood. Kapili has reservoir at upstream. If high precipitation happens dam authority will be bound to release the rainwater once it crosses the reservoir capacity. So reservoir situation should be monitored, so that early warning can be issued to mitigate flood disaster. Also reservoir operation can be augmented to reduce flood marginally.”
Sarma said reducing flood risk would require closer coordination between authorities in the hill states and the downstream districts, along with stronger monitoring and early warning systems.
“There should be a coordination committee for disaster preparedness. To develop an early warning mechanism. Such committee should involve Academician, Reservoir Authority, District Authority of both hilly state and state in plains. Hydrodynamic and hydrological modelling with sensor based input can help in forecasting flood disaster and issuing warning. Installation of rain gauge in the hilly terrain will give better information about flood generation and will help disaster mitigation measures. Awareness generation can help utilizing data through crowd sourcing. Drone based monitoring can also help in identifying critical situations. Immediate steps should be taken collectively for reforestation of the catchment through ecological Management Practices (EMPs) with economically beneficial plants and selecting proper varieties of plant that are suitable for controlling surface erosion. EMPs in addition to vegetation include measures, like retaining wall etc. To support vegetation growth water harvesting structures and suitable plantation and land-use augmentation considering due right of land ownership,” he added.
In 2019, then-Nazira MLA and Leader of the Opposition Debabrata Saikia had filed a public interest litigation (PIL) before the Gauhati High Court seeking a ban on illegal riverbed (sand and stone) mining in the Dikhow river and the smuggling associated with it. The petition had specifically warned that continuous stone extraction would render the Dikhow shallow and heighten the risk of catastrophic floods and erosion in Sivasagar and Nazira — a warning that, in effect, foretold the present crisis years in advance.
On October 21, 2019, the High Court had issued directives to the Assam government, including the formation of a dedicated Mines and Mineral Task Force Battalion under the Department of Mines and Geology to curb illegal mining, and the constitution of a high-power committee to protect the Dikhow’s natural ecosystem and prevent alteration of its course.
“But these directives were never implemented,” Saikia said, adding that he had again cautioned the Assembly in 2022, citing a Water Resources Department report, that continued mining could turn the Dikhow into a “sea-like” expanse during floods.
In August 2025 — a year before the floods — residents of Nazira had publicly alleged that illegal stone and boulder mining in the Dikhow was aggravating erosion around Simaluguri market and Tetaliguri, and that a newly built embankment-cum-road link between Tetaliguri and Masterchuk had already been washed away.
This flood is only the beginning. More disasters are likely to strike Assam in the days ahead. And it is not confined to Nagaland alone — ongoing infrastructure projects such as hydropower schemes, National Highways and railway lines in the neighbouring states of Meghalaya and Arunachal Pradesh are also likely to trigger further disasters in Assam. The reason remains the same: coal mining, stone mining, hill-cutting and tree-felling for development projects.