The author, an award-winning expert on water-related issues, recommends a raft of measures to improve resilience in the event of a flood in the high Himalayas. First, he calls for a robust, basin-wide Disaster Risk Reduction framework. Second, integrating disaster management into India’s Concurrent List, and reforming the National Disaster Management Authority to include regional expertise. Third, halting additional mega dam projects in the Upper Tista Basin. Fourth, framing a regional hydropower policy embedding environmental and climate considerations, with an effective early warning system. Fifth, exploring the concept of “Payment for Ecosystem Destruction” to ensure accountability for upstream degradation impacting downstream communities. Finally, by promoting participatory planning that incorporates local knowledge, a dedicated Tista Disaster Management Authority may promote equity and resilience.
The transboundary Tista River, the fourth largest in Eastern South Asia after the Ganges, Brahmaputra, and Meghna, originates as Chhombo Chhu from the glacial lake Khangchung Chho at 5,280 meters in Sikkim’s northeastern corner (Parsai and Suri 2013, Khawas 2021). It flows through the entire length of Sikkim, carving profusely vibrant Himalayan temperate and tropical river valleys. The river travels 172 kilometres through Sikkim and Darjeeling’s hilly terrain in India, then meanders 98 kilometres across West Bengal’s plains before covering an additional 134 kilometres in Bangladesh, where it joins the Brahmaputra (Khawas 2021). With a total drainage area of 12,159 square kilometres, the Tista basin allocates 83.5 percent of its catchment to India—57 percent in Sikkim (6,930 sq km), 9.2 percent in Darjeeling, West Bengal (1,121 sq km), and 17.3 percent in the plains of Jalpaiguri and Coochbehar, West Bengal (2,104 sq km)—while the remaining 16.5 percent (2,004 sq km) lies in Bangladesh. As it meanders downstream, the Tista receives water from several tributaries on either side of its course, forming a complex and dynamic river basin and a unique eco-region often referred to as the “Tista Eco-region” (Khawas 2015, 2021).
The Tista’s Socio-cultural and Economic Influence
The Tista Basin, a global biodiversity hotspot abundant in natural and bio-resources, acts as a vital ecological sink, and maintains environmental balance on a global scale. Moreover, the basin has historically been home to several social groups in Sikkim, northern West Bengal (India) and Bangladesh (Khawas 2015, 2021). For centuries, it has sustained diverse social groups nurturing rich cultural traditions. Supporting approximately 30.36 million people, the basin ensures regional human security and environmental stability. Starting from the Lepcha tribe, the ethnic Bhutias, and the ethnic Nepalis in the Sikkim-Darjeeling Himalayas, to the agrarian communities of North Bengal and Bangladesh, the region is the source of livelihood for several socio-cultural groups.
The basin has been the ethno-cultural and ethno-religious foundation for many social groups in the Darjeeling-Sikkim region. All the ethnic groups are characterized by specific ecological adaptations as well as by the social organization of the region where they live (Bhasin and Bhasin 1996).
Overview of the Tista River Basin
| Geographical Region | Political/Administrative Region | Basin Area (Sq Km) | Land (%) | Population (%) | |
| Hills | Upper Basin | Sikkim | 6930 | 57.0 | 02 |
| West Bengal (Darjeeling hills) | 1121 | 9.2 | 51 |
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| Plains | Middle Basin | West Bengal (Jalpaiguri and Coochbehar) | 2104 | 17.3 | |
| Lower | Bangladesh | 2004 | 16.5 | 47 | |
| Total | 12159 | 100.0 | 100.0 | ||
Source: Based on the report, State of Environment: Sikkim, 2007 cited in Khawas 2021.
The Tista is the primary source of water for agricultural crops in northern West Bengal and north-western Bangladesh. Besides, other associated livelihood options—to name only a few, such as rafting, sandstone mining, and fishing—have thrived across the basin for a long time. The Tista basin, therefore, provides significant human and environmental security to the geographic milieu of the region (Khawas 2021). Despite Sikkim covering 57 percent of the basin’s area, it is home to only 2 percent of its population, while West Bengal accounts for 51 percent and Bangladesh 47 percent, highlighting an uneven demographic distribution across the basin. The diversity and demographic spread signifies the basin’s critical role in ecological and socio-economic contexts, now increasingly challenged by modern development pressures particularly in its upper catchment also called the Upper Tista Basin (UTB) covering Sikkim, Darjeeling hills, and part of Bengal Duars.
Environment Challenges and Development Pressures
The Upper Tista catchment is a pristine landscape defined by glaciers, glacial lakes, rivers, streams, and natural springs, shaped by intense monsoon rainfall often exceeding 3,000 centimetres annually. The UTB experiences heavy rainfall due to its proximity to the Bay of Bengal (CSE 1991). Measuring just around 65 km east to west, and about 200 km from north to south, the region ranges from foothills (with less than 250 meters), sweltering deep valleys (with a mere 300 meters) to lofty snow peaks such as Kangchendzonga at 8,598 meters (Lama and Khawas 2009). On the north-western side of the region lies the massive 31 km long Zemu Glacier. Two principal mountain ranges of the UTB are the Singilela[1] and Chola. They start in the north and continue in a more or less southerly direction. Between these ranges are the principal rivers, the Tista and its tributary the Rangit, forming the main channels of drainage. These rivers are fed by the monsoon as well as by melting glaciers.
The UTB’s botanical and zoological affluence is truly impressive and profusely fascinating (Lama and Khawas 2009). The dynamic environment supports a rich ecological terrain but faces growing pressures from modern development. Historically, the Upper Tista Catchment fostered a symbiotic relationship between its people and the environment. However, this balance is increasingly threatened by diverse development pressures in recent decades (Khawas 2015, 2021). Over the past four decades, Sikkim’s economy has undergone a remarkable transformation, evolving from a small, agrarian base to a diversified, high-growth model. In 1975, Sikkim’s per capita income was less than ₹2,000, but by 2024, it soared to ₹588,000, surpassing Goa’s ₹575,000. Adjusted for inflation, this represents a 10–15-fold increase over 1975 levels. Similarly, the gross state domestic product (GSDP) expanded from under ₹100 crore in 1975 to approximately ₹52,000 crore by 2024–25, achieving a compounded annual growth rate of 11.5 percent since 2011–12 (Khawas and Bhattacharya 2025).
This economic shift is reflected in the changing contributions of different sectors to Sikkim’s GDP. In the late 1970s, the primary sector, encompassing agriculture and mining, accounted for over 50 percent of the GSDP. By 2022–23, its share had declined significantly to 8.83 percent. In contrast, the secondary sector, driven by industries such as construction, pharmaceuticals, and hydropower, surged from 20 percent in the late 1970s to 62.60 percent by 2022–23. The service sector, which dominated until 2005 with a contribution of 30 percent, grew to 55 percent by 2011, but subsequently fell to 28.58 percent by 2022–23 (NITI Aayog 2025, StatisticsTimes.com 2024, Basnet and Mahapatra 2024). Workforce trends broadly mirror this structural transformation, with a decline in cultivators and a rise in employment in the secondary and tertiary sectors. While the economic diversification has driven prosperity, it poses significant risks to the delicate ecological balance of the Upper Tista Basin, threatening the region’s environmental sustainability.
Disaster Risks in Sikkim and North Bengal
Sikkim-North Bengal region is highly vulnerable to multiple disaster risks, such as earthquakes, landslides, cloudbursts, floods, droughts, and the intensifying effects of climate change in the shape of glacial retreat and the formation of potentially dangerous glacial lakes. In the Sikkim Himalaya, glaciers like Jemu have retreated at approximately 20 meters per year between 1975 and 1990. The Tista basin hosts 266 glacial lakes, 55 of which are classified as potentially dangerous for glacial lake outburst floods (GLOFs) by the International Centre for Integrated Mountain Development (ICIMOD). Further, according to Bahadur (2004) the Tista River is among the swiftest rivers in the Himalayas and carries a sediment yield of 125.1 tons per hectare per year at Anderson Bridge, far exceeding other Himalayan rivers like the Tamur (82.2 t/ha/year), Karnali (67.4 t/ha/year), and Brahmaputra (14.5 t/ha/year). This translates to approximately 98 cubic meters of silt per hectare annually, resulting in a denudation rate of 9.8 millimetres per year, one of the highest globally (CSE 1991). These factors heighten the region’s susceptibility to devastating floods and other geomorphic events, posing significant threats to environmental and human security, that have been repeatedly tested by historical and recent disasters.
Historical and Recent Flood Disasters
Tista River’s complex geology and geomorphology, combined with heavy and often climate-induced monsoon rainfall, makes it highly prone to catastrophic floods and other geomorphic hazards. Historical events signify this vulnerability.
According to Hunter’s Statistical Account of Bengal, the Tista was originally a river of the Ganga basin. The Tista, which at present flows into Bangladesh to meet up with the Brahmaputra, used to flow into the Mahananda and the Ganga in Bihar. In 1787, due to incessant rain followed heavy flood and devastating earthquake, it shifted its course to the Brahmaputra basin. If such sudden river capture occurs today, it will sweep away thousands of villages in a gigantic flash flood (Khawas 2015).
Similarly, floods in 1950, 1955, and 1968 caused widespread devastation. The 1968 flood, driven by intense monsoon rains and landslides, triggered an estimated 20,000 landslides, severed the 50-kilometer Siliguri-Darjeeling road in 92 places, and resulted in approximately 20,000 people dead, injured, or displaced. Bridges were destroyed, and rail traffic halted for 32 days, permanently altering the river’s floodplain (CSE 1991, Khawas 2009).
More recently, the 2015 flash flood, fuelled by torrential rains in Sikkim and Darjeeling, led to surplus discharge from the Gajoldoba Barrage, inundating the downstream. Reportedly, in July 2015 excess unannounced water was released from hydro project dams of Sikkim causing flash floods in villages located on both banks of the Tista downstream in Darjeeling and Jalpaiguri districts of West Bengal (Khawas 2015a). Considering this problem, the chief minister of West Bengal advised the North Bengal development minister to visit Sikkim and apprise the chief minister of Sikkim on the matter so that such release of water can be regulated (The Telegraph 2015, Khawas 2015a). The West Bengal government has been in the process of developing a communication mechanism so that it is informed about such discharge of water. This surely is a pointer to the potential future intra-state disputes in the region (Khawas 2015a). In August 2017 intense monsoon rainfall, combined with water releases from dams and barrages in the Tista and Brahmaputra River systems, caused widespread flooding in northern West Bengal and Bangladesh, displacing over 600,000 people, particularly in northern Bangladesh and northern West Bengal (OCHA 2017). The 2023 glacial lake outburst flood (GLOF) from South Lhonak Lake obliterated the Tista-III Dam, devastating Sikkim, the Darjeeling hills, West Bengal’s plains, and Bangladesh. These recurring disasters highlight the urgent need for improved flood management and preparedness particularly in light of the 2023 catastrophe.
The 2023 Glacial Lake Outburst Flood (GLOF)
On October 3-4, 2023, a catastrophic glacial lake outburst flood from South Lhonak Lake in Sikkim ravaged the Tista River basin. Potentially triggered by a landslide from a collapsing lateral moraine, the flood overwhelmed the Tista-III Dam, causing widespread destruction across Sikkim, the Darjeeling hills, West Bengal’s plains, and Bangladesh. GLOFs are not new to the Himalayas, but their frequency has increased over the past few decades due to climate change accelerating geomorphic processes. When the Tista-III Dam was planned in the early 2000s, engineers relied on limited data, with the highest recorded flood discharge of 680 cubic meters per second (cumecs) from 1973 in Chungthang. Designed to withstand 7,000 cumecs, the dam was unprepared for the rapid changes in glacial and hydrological dynamics. Reportedly, the National Seismic Designing Authority requires 42 years of water data for mega projects, yet Sikkim-Darjeeling, being a backward region, lacks comprehensive water records, fuelling criticism from anti-dam activists about the technical, environmental, and human risks of hydropower projects.
Critics argue, had the dam’s gates been opened on time, the flood’s impact could have been significantly reduced, mitigating downstream devastation. However, due to the absence of an effective early warning system in place, by the time information reached the dam operators, the flood had reportedly struck the dam, forcing them to flee for their lives instead of opening the gates. Furthermore, some argue that without the dam’s presence, the event might not have escalated into a full-scale disaster, raising critical questions about the role of large-scale infrastructure in amplifying risks in the geomorphologically vulnerable upper Tista Basin.
Downstream Impact of the 2023 GLOF
The Tista flood caused widespread devastation affecting Chungthang, Dikchu, Singtam, Rangpo, Melli, Tista Bazar, 29th Mile, and Kali Jhora in the Sikkim-Darjeeling catchment, and Chumukdangi and Tot Goan in Jalpaiguri, West Bengal. The flood disrupted National Highway 10 and reportedly raised the riverbed by 15 to 20 meters between Rangpo and Tista Bazar complicating recovery efforts. It claimed 179 lives, displaced thousands, and affected over 100,000 people, with property and habitation losses exceeding ₹3,000 crore in Sikkim and ₹110 crore in Kalimpong. Notably, while the Tista-III Dam collapsed, the Tista-V Dam at Dikchu withstood the flood, as some gates were opened in time. Had Tista V failed, the consequences could have been catastrophic, potentially destroying downstream dams (TDLP-III at 27th Mile and TDLP-IV at Kalijhora). The tremendous volume of Tista water along with the massive debris flow could have swept through Sevoke Bazar, potentially redirecting the river into the Mahananda river, flooding Siliguri city and endangering the lives of over 1 million people in Siliguri. This near-catastrophe event underscores the Tista’s complex geomorphic history, the critical need for rigorous research, and integrated planning to mitigate future risks.
The October 2023 Tista flood exposed significant weaknesses in India’s disaster management framework, particularly in the Sikkim-North Bengal catchment. The deluge devastated Sikkim, the Darjeeling hills (especially Kalimpong), and West Bengal’s Duars, but responses varied sharply. Sikkim received robust national attention and coordinated relief, while the Darjeeling hills and Duars were largely overlooked, leaving communities to face devastating losses with minimal support. In India, disaster management is primarily a state responsibility with the central government providing guidance and resources. However, central involvement in the Darjeeling hills was minimal, limited to brief mentions by the local member of parliament. Political dynamics likely exacerbated this neglect, as the Trinamool Congress-led West Bengal government avoided declaring the flood a “disaster,” despite Kalimpong’s reported heavy human and non-human damage. Such a declaration would have unlocked central funding, potentially benefiting the opposition Bharatiya Janata Party. The political sidelining highlights the need for a more equitable and robust disaster management framework to ensure fair support across all affected regions.
Strategizing Disaster Risk Reduction for Flood Resilience in the Upper Tista Basin
To address the complex challenges posed by floods and other geomorphic processes, an integrated, basin-wide disaster risk reduction (DRR) plan for the Tista is essential, building on existing sub-regional plans. The inclusion of disaster management in the Concurrent List of the Indian Constitution would empower both central and state governments to legislate and respond effectively. The National Disaster Management Authority (NDMA) requires reform to include subject experts, academics, and regional representatives from Northeast India to address the region’s unique disaster challenges. Comprehensive guidelines for settlements and construction in fragile Himalayan zones are urgently needed to ensure sustainable development.
Regionally, Sikkim, Darjeeling hills, North Bengal plains, and northwest Bangladesh should work together through a shared/common platform to stop additional mega dam projects in the upper Tista catchment. Given Sikkim’s high density of hydropower projects, it should develop a regional hydropower policy that integrates DRR and climate change considerations. Environmental Management Plans (EMPs) and climate change strategies must be embedded in all development projects to minimize environmental risks. An efficient early warning system, as emphasized by the Sendai Framework (2015–2030), is critical to mitigate future risks and enhance resilience across the Tista Basin.
Several critical issues require urgent discussion to ensure the Tista Basin’s long-term resilience. The rapid development trajectory of the Upper Tista Basin, particularly Sikkim’s rapid growth over the past 30–40 years, demands a thorough evaluation to assess its environmental impact and sustainability. The concept of “Payment for Ecosystem Destruction” merits exploration, as downstream regions bear the consequences of upstream environmental degradation, necessitating mechanisms to ensure accountability. A comprehensive regional DRR plan, potentially overseen by a dedicated Tista Disaster Management Authority, could strengthen preparedness and coordination across the basin. Participatory disaster planning, incorporating local and traditional knowledge as advocated by the Sendai Framework, is essential for effective DRR. The Tista Basin should have a robust disaster management framework that prioritizes resilience and equity across all its sub-regions.
Concluding Remarks
The Tista River Basin is a critical ecological and socio-economic lifeline for Sikkim and North Bengal. It faces escalating challenges from floods, glacial lake outburst floods, and development pressures in its upper catchment, as evidenced by the catastrophic 2023 South Lhonak Lake flood. The disaster highlights the urgent need for a robust, basin-wide DRR framework to enhance flood resilience. Integrating disaster management into India’s Concurrent List, reforming the National Disaster Management Authority to include regional expertise, and halting additional mega dam projects in the Upper Tista Basin are essential steps. A regional hydropower policy embedding environmental and climate considerations, alongside an effective early warning system aligned with the Sendai Framework, can mitigate risks. The concept of “Payment for Ecosystem Destruction” merits exploration to ensure accountability for upstream degradation impacting downstream communities. By promoting participatory planning that incorporates local knowledge, a dedicated Tista Disaster Management Authority could promote equity and resilience across the basin’s diverse sub-regions.
Vimal Khawas is a Professor in the Special Centre for the Study of North East India at Jawaharlal Nehru University (JNU), New Delhi. He was conferred the Prof S.R. Basu Memorial Award in November 2025 for his work on water-related issues at the 4th International River Conference of the South Asian Institute for Advanced Research and Development in New Delhi. Previously, he was an Associate Professor in the Department of Peace and Conflict Studies and Management, Sikkim University. He headed the department in 2018-20. He holds an MPlan degree (Urban and Regional Planning) from CEPT University, Ahmedabad, and PhD (Political Geography) from the Political Geography Division, Centre for International Politics, Organisation and Disarmament, JNU. He specializes and works around issues of regional development and planning, disaster management, environmental security, and natural resources management.
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[1] The range separates the Upper Tista Basin and (Eastern) Nepal.
