The Ranganadi River, a sub-tributary of the Brahmaputra, originates in the Himalayan foothills of Arunachal Pradesh at an elevation of approximately 3,400 meters. Spanning a total length of 150 km, it flows for about 90 km through the hilly terrain of Arunachal Pradesh before entering the plains of Assam, where it continues for another 60 km. The river’s catchment area covers roughly 2,941 square kilometers, with 31 percent lying within Assam. It enters Assam at Johing in Lakhimpur district and eventually merges with the Subansiri River at Pokoniaghat. The river’s average annual discharge is estimated at 3.78 BCM (billion cubic meters), translating to a flow rate of around 120 cumec (cubic meters per second).
By nature, the Ranganadi is an aggrading and meandering river. In its upper catchment, numerous tributary streams—both large and small—join the main channel. These streams transport significant loads of silt, sand, stone aggregates, boulders, and gravel, which are deposited along the riverbed as it descends toward the plains.
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Ranganadi Hydro Electric Project
The North Eastern Electric Power Corporation (NEEPCO) constructed a 68-meter-high concrete gravity dam on the Ranganadi River near Yazali in Arunachal Pradesh, approximately 65 km along its course. This structure forms part of Stage I of the 405 MW Ranganadi Hydro Electric Project (RHEP). The dam created a modest reservoir with a total capacity of 7.71 MCM (million cubic meters), of which 4.43 MCM is live storage used for power generation. Water is diverted from the reservoir at a rate of 160 cubic meters per second (cumec) through a 10 km headrace tunnel to the Dikrong River through a 3×135 MW powerhouse.
Prior to impoundment, the Ranganadi’s average annual flow was around 120 cumec, but post-regulation (since 2002), this has declined by 63% to 44 cumec. Conversely, the median annual flow in the Dikrong River has increased by 138%, rising from 86 cumec to 119 cumec, reflecting the diversion’s impact.
Despite its installed capacity, the powerhouse has consistently underperformed due to insufficient inflow, which rarely meets the design requirement of 160 cumec—even during much of the monsoon season. Recognizing this, the Central Electricity Authority (CEA) revised the project’s design energy output downward from 1,874 million units (MUs) to 1,509.69 MUs, based on a 23-year hydrological assessment.
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Impacts of river diversion
The diversion of flow to the Dikrong has also led to the drying up of the Ranganadi’s downstream stretch, particularly the first 10 km below the dam, after which a few minor streams replenish the river. Since the project was conceived in the 1980s—before environmental regulations mandated minimum environmental flow releases—no provision was made to sustain downstream ecosystems or dependent communities. This omission has had severe ecological and socio-economic consequences.
During the monsoon, when inflows exceed 160 cumec and the reservoir reaches its maximum level of 567 meters, excess water is discharged through the spillway gates. However, if inflows surpass 1,290 cumec—the safe carrying capacity of the downstream channel with embankments—flooding occurs in the lower reaches.
To address storage limitations, Stage II of the RHEP was proposed 12 km upstream of the existing dam. It includes a 108-meter-high concrete gravity dam with a reservoir capacity of 523 MCM, intended to support an additional 80 MW power station and regulate flows for Stage I. However, the project has remained stalled since 2013 due to the lack of statutory clearances. According to the CEA’s 2023–24 Annual Report, the Detailed Project Report (DPR) for Ranganadi Stage II (130 MW) was returned to NEEPCO for resubmission after necessary inputs were not fully tied up.
Downstream Flooding and Embankment Vulnerability
The Ranganadi River is flanked by 57.53 km of embankments constructed by the Water Resources Department (WRD), Assam—28.17 km on the left bank and 29.84 km on the right. In the lower plains, flooding and erosion are persistent threats, causing significant damage to life and property. Over the decades, aggradation of the riverbed has led to a gradual rise in bed level, reducing the channel’s capacity. During the monsoon, this results in sudden flood-lift, which rapidly elevates water levels and places immense pressure on both embankments. The annual recurrence of such events reduces the freeboard, increasing the risk of overtopping.
To address this, the flood management scheme in North Lakhimpur district focuses on anti-erosion and river training works. The scheme includes three core components i.e., raising & strengthening of embankment, bank revetment & launching apron and PSC Porcupine for flow deflection and energy dissipation. The scheme is executed by North Lakhimpur Water Resources Division, Assam under the flood management programme. These interventions are implemented by the North Lakhimpur Water Resources Division under the Flood Management Programme (Manish Gupta et al., CSMRS 2018).
Post-Dam Hydrological Changes and Flood Risk
Since the commissioning of the Ranganadi Hydro Electric Project (RHEP) in 2002, the downstream river width has narrowed significantly during non-monsoon months due to the diversion of 160 cumec of flow to the Dikrong basin. This reduction in base flow has fortified settlements within the riverbed, increasing vulnerability during high-flow events.
During the monsoon, when inflows exceed 160 cumec and spillway releases are initiated, the discharge can surpass 1,291 cumec—the safe carrying capacity of the river with embankments in place. Such events lead to overtopping and embankment breaches, inundating villages, towns, and agricultural fields. Once breached, the embankments lose structural integrity and even moderate flows can trigger flooding in the adjacent floodplains.
Recent Flood Havoc in Lakhimpur District
On the night of May 30–31, 2025, intense rainfall coupled with dam-induced releases from the Panyor Hydro Electric Plant (PHEP)—formerly known as RHEP—triggered severe flooding across Lakhimpur district, claiming two lives and leaving thousands stranded. The Ranganadi began to overflow around 1:00 a.m., following the upstream discharge by NEEPCO, inundating 243 villages and prompting round-the-clock rescue operations led by the district administration and SDRF teams.
The situation deteriorated further when the Mora Singra embankment at Atichuk gave way, allowing floodwaters to surge through Khagorigaon. Simultaneously, the ULFA Embankment at Aamtola was breached, submerging over 100 villages in the North Lakhimpur Revenue Circle.
Villages such as No. 1 Dejo Pathar, Pachnoi, Na Bhagania, and Deobil were among the worst affected. Residents were forced to evacuate hastily, abandoning homes, belongings, and food supplies as floodwaters swept through the region. Relief and rescue efforts continue as authorities work to assess the full scale of the devastation. (The Assam Tribune, May 31, 2025)
Flood Attribution and Flow Dynamics
The Panyor Hydro Electric Plant (PHEP) has once again come under scrutiny for its alleged role in downstream flooding. However, flow data from May 29–31, 2025, tells a more complex story. Inflow to the reservoir rose steadily from 54.78 cumec at 3:00 a.m. on May 29 to 257.51 cumec by 1:00 a.m. on May 30, while no water was released downstream during this period. Later that day, inflow surged from 254.11 cumec at 2:00 p.m. to a peak of 1,621.83 cumec by 11:00 p.m., with corresponding outflows increasing from 8.00 cumec to 1,131.50 cumec. After midnight on May 31, both inflow and outflow began to decline. Notably, at no point did the outflow exceed the river’s safe carrying capacity, suggesting that the flooding may not have been directly caused by dam releases alone.
Indiscernible Victims and Uneven Attention
As K.K. Chatradhara poignantly observed in Assam Times (Nov 2017), the 6,000 residents living between the Ranganadi embankments, particularly in the char areas, endure perennial flooding yet remain largely unrecognized as flood-affected. Ironically, when embankments breach and floodwaters spill into their settlements, many feel a sense of relief—not from the flood, but from finally being acknowledged and relocated to relief camps. In contrast, those living outside the embankments experience constant anxiety during high-water alerts, relying on the embankments as their only shield. When these outer areas—especially townships—are impacted, the disaster garners greater media and administrative attention, often due to the higher visibility and intensity of damage.
Embankment Failures and Downstream Flooding
The primary cause of flooding in the downstream reaches of the Ranganadi River is the repeated failure of embankments. In 2025, breaches at the Mora Singra guide bund in Atichuk and the “ULFA Embankment” at Aamtola led to the inundation of 243 villages and the submergence of National Highway 15. These incidents echo earlier failures—in 2008, when 10 breaches spanning 7.8 km occurred near Ujani Khamti, and again in 2017, with breaches at Noinpur (July 9, 10:00 PM) and Bogolijan (July 10, 10:45 PM). Between 2002 and 2014, at least 43 embankment breaches were recorded, primarily due to overtopping, erosion, and inadequate maintenance. Structural deficiencies have compounded the problem. Embankments often lack proper slope gradients—1:2 on the riverside and 1:3 on the countryside—and geo-mattress coverage remains incomplete, even as of 2025. Moreover, embankments constrict the river’s natural flow, leading to increased siltation and raised riverbeds, which in turn reduce channel capacity and heighten the risk of future breaches. ((KK Chatradhara, Hiland Post.com, June 12, 2025).
Siltation, Perception, and the Role of the Dam
Residents downstream argue that the riverbed has become shallower over time due to periodic flushing of silt from the Yazali dam site, a necessary operation for dam maintenance and believe this has intensified the frequency and severity of flash floods.
Ranganadi Dam does not generate water or silt, but the natural river flow and sediment load passes through the dam. As NEEPCO clarified in April 2022, “Floods occurred even before the dam existed. The confusion arises when people assume that any sudden rise in water is due to dam releases. In reality, excess inflow must be discharged downstream. The encroachment into the original riverbed has increased vulnerability. If the river appears dry for most of the year and people settle within its course, they are at risk when natural floods return. The dam’s presence does not create the flood—it only passes what nature delivers.”
Ranganadi vs. Subansiri: A Comparative Perspective
Concerns have been raised that the 2,000 MW Subansiri Lower Hydro Electric Project being constructed by NHPC might cause downstream flooding at a scale five times greater than the 405 MW Ranganadi Project. However, the comparison reveals a different picture. While Ranganadi has a 7.7 MCM reservoir, Subansiri boasts a 1,365 MCM reservoir, with 442 MCM dedicated to flood storage during the monsoon. This enables it to withhold a 10-year return flood of 12,400 cumec for 24 hours.
Additionally, NHPC’s riverbank protection measures have reportedly increased the Subansiri River’s carrying capacity by 15–20%, from 7,000 cumec to a higher threshold along a 60 km stretch downstream. This is supported by the fact that, despite a recorded flow of 8,873 cumec on May 31, 2025, no flooding was reported in the Subansiri basin.
Conclusion
Contrary to public apprehension, the Ranganadi Dam—owing to its limited reservoir capacity—did not cause downstream flooding. With only 4.43 million cubic meters (MCM) of live storage, the dam neither generates water nor silt; it merely passes the natural river flow and sediment load downstream. Even if the entire live storage were released abruptly, the resulting surge would last no more than seven minutes, insufficient to cause sustained flooding.
The increased river discharge observed during the recent flood was primarily due to excessive rainfall in the catchment area, which led to spillway releases. According to NEEPCO, the Ranganadi Hydro Electric Project (RHEP) is equipped with a six-hour lead time Early Warning System, including sirens installed in downstream areas. However, flood-affected residents have reported that these sirens often fail during power outages. To ensure reliability, the system should be backed by battery inverters and solar power. Moreover, effective coordination between the project authority and local administration is essential to issue timely warnings and facilitate evacuation during flood forecasts.
Historical data shows that embankment failures—not dam releases—were the primary cause of downstream flooding in 2008, 2017, and 2025. These embankments must be urgently renovated and reinforced to withstand future flood events.
To enhance flood moderation, the Government of India should prioritize clearance of the proposed Ranganadi HEP Stage-II, which includes a fairly large storage reservoir capable of regulating flows and mitigating downstream flood risks.