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Teesta River
তিস্তা(Tistā)
Teesta in Siliguri, West Bengal
Path of the Teesta
Map
Location
Country
States
Important Bridges
District's
Towns & Cities
Physical characteristics
SourcePauhunri, Zemu Glacier, Gurudongmar Lake, Tso Lhamo Lake
 • locationSikkim, India
 • elevation7,128 m (23,386 ft)[1]
MouthJamuna River
 • location
Phulchhari Upazila, Gaibandha, Bangladesh
 • coordinates
25°30′20″N 89°39′57″E / 25.50556°N 89.66583°E / 25.50556; 89.66583
Length414 km (257 mi)[1]
Basin size12,540 km2 (4,840 sq mi)[2]
Basin features
Tributaries 
 • left
 • right

Teesta River (Bengali:তিস্তা,Tistā)is a 414 km (257 mi) long river that rises in the Pauhunri Mountain of eastern Himalayas, flows through the Indian states of Sikkim and West Bengal and subsequently enters Bangladesh through Rangpur division. In Bangladesh, it merges with Jamuna River which after meeting some other major rivers of the Bengal delta finally falls into the Bay of Bengal.[1] It drains an area of 12,540 km2 (4,840 sq mi).[2] In India, the Teesta River flows through Mangan District, Gangtok District, Pakyong District, Kalimpong district, Darjeeling District, Jalpaiguri District, and Cooch Behar District as well as the cities of Rangpo, Jalpaiguri, and Mekhliganj. In Bangladesh, it flows through Lalmonirhat District, Rangpur District, Kurigram District and Gaibandha District. It joins the Jamuna River at Phulchhari Upazila in Bangladesh. 305 km (190 mi) of the river lies in India and 109 km (68 mi) in Bangladesh. The Teesta is the largest river of Sikkim and second largest river of West Bengal after the Ganges.

Course

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Teesta River near Gangtok
Teesta River near Gangtok
National Highway 10 winds along the banks of the Teesta River near Kalimpong.

The Teesta River originates from Teesta Khangtse Glacier, west of Pahunri (or Teesta Kangse) glacier above 5,400 m (17,700 ft), and flows southward through gorges and rapids in the Sikkim Himalaya.[3]

It is fed by streams from Tso Lhamo Lake, Gurudongmar Lake and rivulets arising in the Thangu Valley, Yumthang Valley of Flowers, Dikchu and Donkha mountain ranges. The river then flows past the towns of Chungthang, Singhik, Mangan, Dikchu and Makha, where some major tributaries like Kanaka river join it, and reaches Singtam, where it is spanned by the scenic Indreni Bridge. Also in Singtam, a large tributary of the Teesta called Ranikhola joins and then descends towards Bardang, Majitar, Mining where it is spanned by Rangpo - Mining Teesta Bridge and reaches the city of Rangpo where Rangpo River, the second largest tributary of the Teesta joins. From here river Teesta forms the border between Sikkim and West Bengal up to Teesta Bazaar via Melli. Just before the Teesta Bridge, where the roads from Kalimpong and Darjeeling join, the river is met by its largest tributary, the Rangeet River.[4]

After this point, Teesta River changes its course southwards flowing into West Bengal and some more tributaries like Relli River, Riyang River, Geil Khola etc joins. Than the river hits the plains at Sevoke, 22 km (14 mi) northeast of Siliguri, where it is spanned by the Coronation Bridge and Sevoke Railway Bridge linking the northeast states to the rest of India. After crossing Sevoke, the river is fed by some small tributaries like Chel Khola, Neora Khola, Leesh Khola etc, and then reaches Gajoldoba where there is Teesta Barrage. The river then flows through Jalpaiguri, where Jalpaiguri Teesta Bridge links Northeastern States. The river flows further down through Mekhliganj and Haldibari in Cooch Behar district, where it is spanned by Joyee Setu, the longest roadway bridge of West Bengal with length 3.8 kilometres.

Here the Teesta completes its journey in India and enters Bangladesh in Dahagram, Rangpur Division. The river descends towards Barakhata, where there is Teesta Barrage of Bangladesh. Further it reaches Rangpur and Lalmonirhat town in Lalmonirhat District and Nilphamari District. After crossing Tepamadhupur Bazaar, Nagrakura Bazaar, Sundarganj and Bozra Bazaar, river Teesta joins Brahmaputra River at Haripur Port near Gaibandha, Phulchhari Upazila in Kurigram District of Bangladesh.[5]

Gajoldoba Barrage on Teesta River

Geography

[edit]

Through its course, the Teesta River has carved out ravines and gorges in Sikkim meandering through the hills with the hill station of Kalimpong lying just off the river and the city of Siliguri at the foothills of Himalaya. Variegated vegetation can be seen along this route. At lower elevations, tropical deciduous trees and shrubs cover the surrounding hills; alpine vegetation is seen at the upper altitudes. The river is flanked by white sand which is used by the construction industry in the region. Large boulders in and around the waters make it ideal for rafting.

During the monsoon, Teesta River distends its banks, both in size and turbulence. Landslides in this region often dam up parts of the river in this season. It has also caused severe flooding, multiple times during monsoon in Bangladesh.[6][7]

Changes in course of rivers

[edit]
James Rennell's 1776 map shows an earlier flow of the Teesta meeting the Ganges in three channels before a devastating flood in 1787 changed its course

In the past, the Teesta flowed south from Jalpaiguri in three channels: the Punarbhaba, Atrai and Karatoya Rivers. The Teesta changed course as a result of the flooding of 1787, turning southeast to become part of the Brahmaputra.[8]

Hydroelectric projects and barrages

[edit]
Hydel project on the Dikchu, a tributary of Teesta
The Teesta Barrage is a major irrigation project in Bangladesh, in Lalmonirhat District. Construction started 1979 and was completed in 1997–98.[9]

India has an estimated total hydroelectric power potential of 84 GW at 60% load factor. Of this, Sikkim's potential share is 2.9%, or about 4.29 GW.[10]

The successfully completed major projects and dams of Teesta River System are as follows:

The other three completed projects are significantly smaller and minor—Lower Lagyap, Upper Rongni Chhu and Mayang Chhu projects.[10]

Water sharing challenge

[edit]

Disputes over the appropriate allocation and development of the water resources of the river have remained a subject of conflict between India and Bangladesh for almost 35 years, with several bilateral agreements and rounds of talks failing to produce results.[26][27][28]

Negotiations have been going on since 1983.[29][30][31] In 1983, a temporary arrangement was reached: Bangladesh would receive 36%, India 39% while, and the remainder would remain unallocated.[30] Both countries signed a water sharing treaty in 1996 which would look into water sharing between the two countries in general following the Ganges water dispute.[30] The water sharing remains a challenge.[30]

In recent developments, Bangladesh's interim government water advisor, Syeda Rizwana Hasan, told news agency PTI (Press Trust of India) on September 1, 2024, that both upper and lower riparian countries must follow international water-sharing principles. If a treaty remains out of reach, it could mark a significant shift in India-Bangladesh relations if diplomacy fails.[32]

Seismic concerns

[edit]

Teesta River area is in the seismically active Zone-V and has experienced micro-seismic activity. According to India's Ministry of Environment & Forests, the Teesta River dam projects have been approved with the requirement that they adopt suitable seismic coefficient in the design for the dam, tunnel, surge shaft and power house. The projects are cascaded over the length of the river, do not store large amounts water, have small reservoirs, and therefore the projects are expected to have very low risk from the reservoir induced seismicity in the area.[10]

Climate and tectonics

[edit]

The Teesta River has preserved good imprints of climatic and tectonics along its valleys and catchments. The interrelationship between climate, erosion, deposition and tectonic activities is not properly understood to date. These are being studied.[3][33][34]

2023 Teesta Flash Floods

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Main article: 2023 India flash floods

See also

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Teesta River

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from Grokipedia
The Teesta River is a major transboundary tributary of the Brahmaputra River, originating from glacial sources in the eastern Himalayas of Sikkim, India, at elevations around 5,200 meters, and flowing approximately 414 kilometers southward through Sikkim and West Bengal before entering Bangladesh, where it merges with the Jamuna River, draining a basin of about 12,540 square kilometers with 83% in Indian territory. The river supports critical irrigation for agriculture in northern Bangladesh and powers multiple hydroelectric projects in Sikkim, including the Teesta-V dam, though it is highly susceptible to flash floods and glacial lake outburst events, as evidenced by the devastating 2023 flood that damaged infrastructure like the Teesta-III powerhouse. A persistent controversy surrounds the equitable sharing of its waters, with Bangladesh receiving minimal dry-season flow due to upstream Indian barrages such as Gajoldoba, prompting demands for a bilateral treaty that India has resisted amid domestic political pressures from West Bengal, while recent Bangladesh initiatives involving Chinese investment in river management have heightened geopolitical frictions.

Physical Characteristics

Course and Morphology

The Teesta River originates from the Khangchung Chho (also known as Tso Lhamo Lake or Cholamu Lake), situated at an elevation of 5,280 meters in the northeastern corner of , , near the Indo-Tibetan border. It initially flows as the Chhombo Chhu, a stream, before coalescing into the main Teesta channel in the . The river's total length measures approximately 414 kilometers, with roughly 293 kilometers traversing Indian territory through and , and the remaining 121 kilometers within . From its high-altitude source, the Teesta descends southward through steep Himalayan gorges in , passing key locations such as Mangan, Dikchu, and , where it receives tributaries like the and Dik Chu rivers. In this upper reaches, the river cuts through rugged terrain with high gradients, forming narrow valleys and supporting hydroelectric infrastructure. Emerging from the hills near in , it transitions into the alluvial plains around Siliguri and districts, where the gradient flattens and the channel widens. The river then flows southeast, crossing the India-Bangladesh border at Gozoldoba, and continues through northern Bangladesh's , meandering across floodplains before its confluence with the near Chilmari in . Morphologically, the Teesta exhibits a braided channel pattern, particularly in its lower and middle sections, characterized by multiple shifting anastomosing channels, mid-channel bars, and frequent avulsions driven by high loads from glacial and hillslope . The riverbed comprises fine to medium sands in braided reaches, with dynamic planform changes including formation ('chars') and channel bifurcation, exacerbated by seasonal floods and tectonic influences. In the upper mountainous course, it displays straighter, incised morphology with steeper slopes and boulder-strewn beds, transitioning to sinuous-to-braided forms in the plains where ratios indicate moderate meandering interspersed with braiding intensity. Overall, the river's morphology reflects Himalayan , with historical shifts documented over decades due to seismic activity, floods, and anthropogenic interventions like barrages.

Basin and Tributaries

The Teesta River basin encompasses approximately 12,159 km², of which 10,155 km² (83%) lies in —primarily in (6,930 km²) and (3,225 km² combined hilly and plain areas)—and 2,004 km² (17%) in . The basin's transitions from steep Himalayan uplands above 3,000 m , where over 59% of the catchment is located, to lower alluvial plains, with 4,108 km² in flat terrain supporting agriculture and settlements. This division influences and water availability, as upstream mountainous sub-catchments contribute high seasonal runoff while downstream areas experience channel braiding and flooding. The river network features a dendritic pattern in the upper basin, fed by glacial melt and monsoon precipitation, transitioning to braided channels downstream. Major tributaries join primarily in the Sikkim and West Bengal segments, augmenting flow before the river enters Bangladesh at Phulbari. Left-bank tributaries, draining eastern slopes, include the Lachung Chu (originating near Sebu Tso lake), Chakung Chu, Dik Chu (site of hydroelectric developments), Rani Khola, and Rangpo Chu. Right-bank tributaries from western slopes comprise the Zemu Chu, Rangyong Chu, and the Rangit River, the latter being the largest with origins in Sikkim's Dikchu area and significant discharge contributions during monsoons. Hydrological modeling often delineates the basin into 43 sub-basins for runoff analysis, highlighting variability in erosion-prone upper reaches versus sediment-depositing lower ones. inflows account for much of the Teesta's average annual runoff of 60 billion cubic meters, with 90% occurring in the June–September period, underscoring the basin's reliance on upstream and glacial sources for downstream .

Hydrology and Flow Regimes

The Teesta River's is characterized by extreme seasonal variability typical of trans-Himalayan rivers, with flows driven primarily by southwest rainfall and contributions from glacial melt in its upper catchment. The average annual runoff totals approximately 60 billion cubic meters, equivalent to a mean discharge of about 1,900 cubic meters per second (m³/s), though this varies significantly by gauging station due to tributaries and extractions. Over 90% of the annual flow—roughly 54 billion cubic meters—occurs during the period from June to October, reflecting the catchment's dependence on intense seasonal exceeding 2,000 mm annually in much of the basin. Dry season flows, spanning December to May, constitute less than 10% of the annual total, with historical averages at downstream points like the Bangladesh border gauged at 100–170 m³/s before major upstream interventions. This yields a dry-to-wet season variation ratio of approximately 1:10, enabling flood peaks during —such as the 16,330–17,300 m³/s recorded at Teesta Bazar and during the 1968 extreme event—but risking near-depletion in lean periods without storage. Monsoon hydrographs show rapid rise and fall, with maximum monthly contributions (May–October) accounting for 77.93% of mean annual discharge, amplified by the steep gradient and unglaciated lower basin's quick response to orographic rainfall. Glacial inputs from sources like the Tista Kangse peak in pre-monsoon melt (April–June), sustaining before dominance, while the overall regime exhibits high interannual variability tied to influences on regional precipitation. Upstream dams, such as those in , have modified regimes by attenuating peaks and augmenting dry-season releases via run-of-river operations, reducing downstream variability but introducing diurnal fluctuations from power generation peaking. These alterations, documented in hydrological assessments, underscore the river's sensitivity to anthropogenic storage, with pre-dam dry-season flows at border points historically 36% higher than post-1990 averages in some analyses.

Geological and Environmental Context

Tectonic and Seismic Features

The Teesta River basin lies within the Eastern Himalayan orogen, where ongoing convergence between the Indian and Eurasian plates at rates of approximately 40-50 mm per year drives compressional , thrust faulting, and rapid uplift. This tectonic regime has structured the basin across multiple tectonostratigraphic zones, including the Higher Himalayan Crystalline Sequence, Lesser Himalayan Sequence, and Sub-Himalayan Siwalik Group, with the river incising through these units from its headwaters near Pauhunri Mountain southward. Major structures such as the (MCT) and Thrust bound segments of the basin, facilitating differential uplift and influencing sediment provenance, where Lesser Himalayan lithologies dominate contributions despite their mid-crustal position. Active tectonics along these features promote high erosion rates, estimated at 1.7 ± 0.5 mm/year across the basin, enhancing the river's sediment load and channel instability. The Teesta River aligns with the Teesta lineament, interpreted as a fault zone that intersects the Fault in , creating curvilinear patterns that amplify geomorphic response to deformation. This structural framework reflects oblique convergence in the eastern syntaxis, where strike-slip components interact with north-south , leading to localized basin asymmetry and transient knickpoints in the river profile. Such features underscore causal links between plate-scale and fluvial dynamics, with thrust reactivation episodically elevating relief and rerouting drainage. Seismically, the upper Teesta basin in occupies Zone IV of India's seismic zoning code (IS 1893), denoting high hazard potential from proximity to active Himalayan thrusts and local faults, though moderated relative to central Himalayan segments. The region records frequent microseismic events tied to fault creep and stress accumulation, with potential for great earthquakes given the in the eastern arc. The Mw 6.9 Sikkim earthquake of September 18, 2011, epicentered near the Teesta headwaters at 27.73°N, 88.06°E, exemplifies this vulnerability, triggering over 200 landslides that blocked tributaries, damaged Teesta hydroelectric projects (including Teesta-V), and deposited debris flows into the main channel, temporarily damming and altering flow regimes downstream. Such events highlight how seismic shaking exacerbates slope instability in steep, glaciated terrain, compounding flood risks without evidence of systematic bias in official hazard assessments.

Historical Channel Shifts

The Teesta River, traversing the in the plains, has experienced multiple channel avulsions due to sediment aggradation, flooding, and topographic gradients that favor sudden shifts in braided, hyper-avulsive systems. Historical reconstructions indicate successive reorientations, transitioning from an early role to the Brahmaputra, then to the Meghna, and by the mid-18th century to the , reflecting the dynamic interplay of fluvial processes in the region. The most documented avulsion occurred in late August 1787, triggered by intense rains that formed a bar—comprising sand, pebbles, and accumulated timber—near Cantamarry, potentially exacerbated by a upstream . Prior to this event, the river followed a southerly path from through western Dinajpur, joining the system. Post-avulsion, it abruptly redirected southeastward, reoccupying the vestigial, sinuous "Teestah Creek" channel and merging with the Brahmaputra (Jamuna) near Chilmary in present-day , drastically altering regional hydrology and settlement patterns. Evidence for this shift derives from James Rennell's surveys and maps (1765–1777), which depict the pre-avulsion configuration, corroborated by archival correspondence, including District Collector McDowall's report dated 2 September 1787 detailing the flood's immediate impacts. Francis Buchanan Hamilton's field diaries (1807–1811) further confirm the stabilized new course, noting enlarged channels and overbank flooding. This 1787 event was independent of contemporaneous Brahmaputra avulsions, which stemmed from the larger river's lateral erosion into subsidiary channels like the Konayi, rather than Tista inflow.

Climate and Glacial Influences

The Teesta River basin experiences a -dominated climate, with the South West from June to September supplying the majority of and driving peak discharges. Annual rainfall varies significantly with , ranging from about 2,000 mm in the foothills to over 5,000 mm in the higher Himalayan zones, contributing 80-90% of the river's annual flow volume of approximately 60 billion cubic meters. Dry winters and pre-monsoon periods see minimal rainfall, resulting in low flows reliant on non-meteorological sources. Glaciers in the North Sikkim Himalayas, particularly the Teesta Khangse Glacier, form the river's headwaters, feeding it through glacial and associated lakes such as Khangchung Chho. This sustains base flows during the non-monsoon season, comprising 12-18% of annual discharge as recorded from 2000 to 2018, while contributes an additional 58-64%. The basin's glaciers, covering a small fraction of the area but critical for hydrological stability, respond to temperature variations, with higher melt rates during warmer periods enhancing dry-season reliability. Climate change exacerbates glacial influences through accelerated retreat and altered precipitation patterns, with Teesta basin glacier areas diminishing by roughly 5% in recent assessments. Rising temperatures have reduced glacial melt contributions while promoting glacial lake expansion, heightening risks of outburst floods; the October 2023 South Lhonak Lake GLOF, triggered by glacial instability, devastated infrastructure along the river and underscored vulnerabilities to such events. Projections indicate potential shifts in seasonal flows, with initial increases from melt followed by declines as ice reserves deplete, compounded by variable monsoon intensities.

Infrastructure and Resource Development

Hydroelectric Dams and Power Generation

The Teesta River, originating in the eastern Himalayas and descending steeply through Sikkim, supports multiple run-of-river hydroelectric projects that exploit its high hydraulic head for power generation. These facilities, primarily developed in India, contribute significantly to the region's renewable energy output, with Sikkim harnessing the Teesta basin's potential estimated at over 8,000 MW. Installed capacities range from smaller cascade schemes to large-scale plants exceeding 1,000 MW, operated by entities like the National Hydroelectric Power Corporation (NHPC) and private developers such as Teesta Urja Limited. Teesta-V, a 510 MW (3 x 170 MW) facility developed by , is situated near in East on the main Teesta stem. This run-of-river project features a 82-meter-high gravity dam and diverts water through a 10.5 km headrace to generate power via Pelton turbines, with initiated in 1999 and units becoming operational between 2008 and 2010. It supplied approximately 2,500 GWh annually prior to recent disruptions but sustained severe damage from a massive on August 19, 2024, near the powerhouse, halting operations and requiring extensive repairs. Teesta-III, Sikkim's largest hydroelectric project at 1,200 MW (4 x 300 MW), is located at in North and was developed by Teesta Urja Limited under a build-own-operate-transfer model. Commissioned in 2017 after delays from geological challenges and financing issues, it utilizes an 800-meter gross head via a 60-meter and underground , targeting annual generation of around 4,500 GWh. The plant was severely impacted by the October 2023 (GLOF) from , which destroyed key infrastructure including the headrace tunnel and desilting arrangements, rendering it non-operational as of 2025. Downstream in , the Teesta Low Dam-III project, a 1,200 MW run-of-river scheme by at Rambi in , features four 300 MW units commissioned progressively from January to March 2013. It draws from the Teesta's flow post-Sikkim cascades, employing a 46-meter-high barrage and 16.75 km headrace tunnel for power evacuation to the northern grid. This facility remains operational, contributing to via linked barrages like Gajoldoba while generating over 5,000 GWh yearly under variable flows. Several smaller cascade projects, such as Teesta Stage-I (47 MW) and Stage-IV (under development), augment the basin's output, with cumulative operational capacity from Teesta projects exceeding 2,800 MW as of 2023 prior to damages. These installations face challenges from high loads, seismic activity, and climate-induced variability, prompting investments in desilting chambers and monitoring systems to sustain efficiency above 80%.

Barrages, Irrigation, and Water Management

The Gajoldoba Barrage, located in , , , approximately 16 km downstream from the Railway Bridge, serves as the primary structure for water diversion on the Indian side of the Teesta River. Constructed as part of the Teesta Barrage Project initiated in 1975, the barrage spans 0.93 km and facilitates across 922,000 hectares in six northern districts of , with an annual potential of 527,000 hectares in the first sub-stage at 154% intensity. The project includes two off-taking canals, five main canals, and distributaries, enabling water supply for agriculture during dry seasons, though it also generates 67.50 MW of . In , the Teesta Barrage, situated in near the Dalia point, was constructed to support the Teesta Barrage Irrigation Project (TBIP), with building commencing in 1979 and completion in 1997–98. This facility diverts up to 280 cubic feet per second through canals for across three cropping seasons in northern regions, covering extensive command areas via a network of primary and secondary channels. The barrage aims to enhance in flood-prone and drought-affected zones, though operational challenges including and maintenance issues have periodically reduced efficiency. Water management along the Teesta involves coordinated yet contentious diversions from these barrages, with India's Gajoldoba structure reportedly channeling up to 85% of dry-season flow into reservoirs and canals, significantly altering downstream availability in . Infrastructure includes river training works, embankments, and efforts to mitigate and flooding, but erratic releases from upstream barrages have exacerbated channel instability and sediment deposition. Recent initiatives in explore pipelines to extend reach without further basin-wide diversions, aiming to optimize limited amid seasonal variability.

Water Allocation Conflicts

India-Bangladesh Transboundary Disputes

![Gajoldoba Barrage on Teesta river in Gajoldoba][float-right] The Teesta River disputes between India and Bangladesh primarily revolve around the equitable sharing of its waters, particularly during the dry season from January to May, when flows at the border drop to as low as 1,000 cubic meters per second (cusecs). India, holding 83% of the river's 12,540 square kilometer catchment area, has developed upstream infrastructure including the Gajoldoba Barrage completed in 1996, which diverts water for irrigation in northern West Bengal, reducing downstream flows into Bangladesh by an estimated 3,000-4,000 cusecs. Bangladesh, with only 17% of the basin but relying on the river for irrigating 1.83 million hectares and supporting 7.3 million people, argues that these diversions exacerbate droughts, soil degradation, and salinity intrusion in its northern regions. Negotiations for a water-sharing agreement began in the 1970s shortly after Bangladesh's independence, with an ad hoc arrangement in 1983 allocating 39% of the dry-season flow to India, 36% to Bangladesh, and leaving 25% unallocated. Progress stalled in 2011 when a proposed treaty—offering India 42.5% and Bangladesh 37.5%—failed to materialize due to opposition from West Bengal's state government, which cited insufficient water for local agriculture and flood control needs. Bangladesh maintains that without a binding treaty, upstream Indian withdrawals violate riparian principles and hinder its food security, as the Doani Barrage in Bangladesh depends on adequate inflows that are often insufficient for intended irrigation of 92,000 hectares. As of 2025, no comprehensive exists, with bilateral talks under the Joint Rivers Commission remaining deadlocked amid domestic political pressures in and emerging geopolitical complications. 's 2024 announcement of a China-backed Teesta River Comprehensive and Restoration Project, valued at $1 billion, has heightened tensions, prompting protests in over potential debt risks and Indian concerns regarding upstream and influence in the shared basin. Indian officials have emphasized that any resolution requires mutual benefit-sharing beyond mere allocation, including joint flood management and cooperation, while critiquing 's external partnerships as undermining trust-based . The absence of agreement continues to strain relations, with facing recurrent water shortages that empirical data links directly to upstream gauging stations showing diminished releases.

Interstate Tensions Within India

Interstate tensions over the Teesta River primarily arise between and due to upstream hydroelectric developments in Sikkim reducing dry-season water flows available for in northern . Sikkim, where the river originates, has pursued aggressive hydropower expansion, with 26 projects operational or planned on the Teesta aimed at harnessing up to 5,000 MW of capacity, prioritizing power generation over consistent downstream releases. These projects, including the 1,200 MW Teesta-III near , impound water for turbine operations, leading to diminished flows during lean periods when evaporation and seepage further exacerbate losses. Downstream in , the Gajoldoba Barrage, completed in 1975, depends on Teesta inflows to irrigate agricultural lands in the northern districts but has underperformed, covering only approximately 66,000 hectares out of an intended 922,000 hectares due to insufficient water volumes. This shortfall has fueled grievances among farmers and officials, who attribute reduced availability to Sikkim's dam regulations that favor peak power releases over steady flows, straining interstate relations without a binding water-sharing pact. Sikkim's hydropower policy, emphasizing run-of-the-river schemes, has been criticized for inadvertently prioritizing state revenue from electricity sales—contributing significantly to Sikkim's —over equitable riparian considerations for West Bengal's agrarian needs. Environmental incidents have intensified these frictions; for instance, landslides in North in August 2016 blocked a Teesta , forming a temporary lake that posed risks to both states, highlighting vulnerabilities from upstream alterations. Seismic activity in the region, including the , has been linked by critics to cumulative dam-induced stresses, amplifying 's concerns over flood-prone river behavior downstream. Absent formal interstate mechanisms like those under the Interstate Water Disputes Act, 1956, negotiations remain ad hoc, with advocating for mandatory minimum releases from reservoirs to sustain irrigation commands amid growing hydropower proliferation.

Ecological and Societal Effects

Flooding Patterns and Disaster Events

The Teesta River exhibits recurrent flooding primarily during the season from June to September, when heavy rainfall in its Himalayan catchment—exacerbated by the steep and high loads—results in rapid rises in levels and downstream inundation. These events are driven by intense , often exceeding 200-300 mm in short durations in upper , leading to flash floods characterized by high-velocity flows, bank erosion, and debris mobilization. In the lower basin spanning and , floods cause widespread agricultural submergence, with historical peak discharges at stations like Dalia reaching levels that inundate thousands of hectares of cropland annually. Flash floods from localized cloudbursts in the upper reaches amplify risks, as loose glacial till and tectonic contribute to landslides that choke channels and trigger sudden surges. Major historical flood events underscore these patterns, with cloudbursts documented in 1950, 1968, 1969, 1972, 1977, and 1978 causing extensive infrastructure damage in , including bridges, roads, and settlements, alongside economic losses from disrupted connectivity and . The 1968 event, in particular, devastated key crossings and prompted early assessments of river channel instability. In , maximum water flows at gauging stations like Kaunia and Dalia showed increasing trends from 1985 to 2006, correlating with heightened flood frequency and severity during monsoons, though minimum flows declined, indicating drier inter-monsoon periods. These floods have historically led to crop failures, particularly of paddy and , with deposition altering fertile floodplains. A notable post-monsoon anomaly occurred in October 2021, when an extreme flash flood—triggered by prolonged heavy rains—marooned approximately 136,000 people across the basin, destroyed thousands of acres of crops including aman paddy and maize, and caused significant erosion compared to prior events. This event highlighted vulnerabilities in transboundary flow management, as upstream surges overwhelmed embankments without glacial outburst involvement. Overall, flood damages in the Teesta basin reflect causal factors like deforestation-induced soil erosion and seismic loosening of slopes, rather than solely climatic variability, with Sikkim records showing cumulative losses in the millions from recurrent inundations.

Biodiversity and Habitat Alterations

The Teesta River basin supports a diverse array of aquatic and riparian species, characteristic of Himalayan riverine ecosystems. Studies have documented at least 45 fish across 17 families in the lower reaches, including threatened hill-stream and catfishes such as and Sisor rabdophorus, many of which rely on seasonal migrations for spawning. diversity includes distributed along elevational gradients in the Teesta and Rangeet valleys, with herpetofauna adapted to forested riparian zones. Avian indicators of river health, such as the (Myophonus caeruleus) and White-capped Water Redstart (Phoenicurus leucocephalus), inhabit the upper basin, signaling intact and stream habitats. Terrestrial encompasses , reptiles, and mammals in valley forests, though comprehensive basin-wide inventories remain limited. Habitat alterations stem primarily from hydroelectric dams and barrages, which fragment riverine corridors and disrupt longitudinal connectivity essential for migratory species. Upstream projects like the Teesta-V dam have blocked fish migration routes, leading to population declines in rheophilic species dependent on upstream spawning grounds, with environmental impact assessments indicating degraded fish habitats across the basin. Reduced downstream flows from structures such as the Gajoldoba Barrage exacerbate siltation and channel incision, diminishing wetland extents in Bangladesh and altering benthic macroinvertebrate communities that support food webs. These interventions, combined with altered sediment transport, have fragmented ecological zones, favoring invasive or tolerant species over endemics in the Indo-Burma biodiversity hotspot. Extreme events, including the 2023 glacial lake outburst flood (GLOF), have further modified habitats through scour and deposition, temporarily boosting macroinvertebrate diversity via habitat renewal but causing long-term shifts in aquatic morphometry and fish assemblages. Overall, infrastructure-induced flow regime changes pose the dominant threat, with peer-reviewed analyses underscoring irreversible losses in migratory fish stocks and riparian vegetation without compensatory measures like effective fish passes, which have proven inadequate in similar Himalayan systems.

Impacts on Local and Indigenous Populations

Indigenous Lepcha and communities in have opposed hydroelectric projects on the Teesta River since at least , citing threats to their ancestral lands, , and tied to the river. Organizations like the Affected Citizens of Teesta (ACT), formed by indigenous Sikkimese, have campaigned against such as Teesta III and Teesta VI, arguing that these projects endanger Lepcha sacred sites and traditional livelihoods dependent on the river's flow. Protests have led to the scrapping of four proposed dams, though two have been constructed, with ongoing concerns over seismic risks and displacement of local populations. The 2023 glacial lake outburst flood (GLOF) in Sikkim devastated communities along the upper Teesta, destroying the Teesta III dam and displacing thousands, including indigenous groups in and nearby villages. At least 55 people died, 74 went missing, and over 7,025 were displaced across , , and downstream areas, exacerbating vulnerabilities for river-dependent populations already facing land loss from dam reservoirs. In , projects like the Teesta Low Dam Project Stage III have induced displacement, altering socio-economic structures in communities reliant on agriculture and riverine resources. Downstream in , water diversions from upstream barrages like Gajoldoba have caused severe shortages, , and loss, severely impacting communities' livelihoods in the Teesta basin. catches have declined, with only 42 recorded in recent surveys compared to higher historical diversity, forcing fishers to shift occupations amid reduced river flow during dry seasons. Agricultural production has suffered, with increased costs and crop failures affecting millions of farmers in northern districts, where the river's erratic flow from releases has also eroded and displaced riverine dwellers.

Contemporary Issues and Events

2023 Glacial Lake Outburst Flood

On the night of October 3–4, 2023, a (GLOF) originating from in North triggered catastrophic flooding along the Teesta River basin. The sudden breach released an estimated volume of water and debris that surged downstream, eroding riverbanks and amplifying the flood's destructive force over a distance of approximately 385 km, extending into . This event was driven by the rapid expansion of , one of the largest and fastest-growing glacial lakes in the region, fueled by glacier retreat from anthropogenic climate warming and permafrost thaw, which destabilized the moraine dam containing the lake. A possible triggering mechanism included a or ice impacting the lake, though and field analyses confirm the lake's volume had increased by over 40% since 2020 due to melting from the adjacent Guruung and South Lhonak glaciers. The floodwaters reached the Teesta III hydroelectric near , approximately 68 km downstream from the lake, around midnight, overwhelming the structure before spillway gates could be fully opened. The 1,200 MW , a major on the Teesta River, was completely destroyed within minutes, releasing additional water and that exacerbated downstream inundation. Water levels in the Teesta River rose by up to 20 feet (6.1 m) in affected areas, causing breaching of embankments, scouring of bridges, and deposition of massive debris flows that blocked roads and disrupted connectivity across and northern . The resulted in at least 55 confirmed deaths and 74 people missing, primarily in , with over 88,400 individuals displaced and economic losses estimated in billions of rupees from damage alone. Hydrological modeling indicates the peak discharge at the breach site exceeded 4,000 cubic meters per second, with sediment-laden flows altering the Teesta's channel morphology and depositing thick layers of boulders and that persist as barriers to navigation and . While some analyses suggest upstream developments may have contributed to localized vulnerabilities, peer-reviewed assessments attribute the GLOF's initiation primarily to climatic factors rather than dam operations, emphasizing the need for enhanced glacial lake monitoring in the . The event highlighted systemic risks in the Teesta basin, where accelerating glacier melt—evidenced by a 10–15% areal reduction in regional glaciers over the past decade—amplifies GLOF potential, underscoring gaps in early warning systems despite prior identifications of South Lhonak as a high-hazard lake.

Reconstruction Efforts and Ongoing Negotiations

Following the October 4, 2023, glacial lake outburst flood (GLOF) from South Lhonak Lake, reconstruction efforts in the Teesta River basin prioritized restoring hydropower infrastructure damaged or destroyed along the river in Sikkim. The 1,200 MW Teesta-III hydroelectric dam at Chungthang was completely swept away, prompting the National Hydroelectric Power Corporation (NHPC) to propose rebuilding it with enhanced safety measures. On January 29, 2025, India's Ministry of Environment, Forest and Climate Change (MoEFCC) granted approval for the reconstruction, overriding concerns about seismic vulnerabilities and inadequate glacial lake monitoring, despite pending detailed project reports and cumulative impact assessments. A central expert committee, appointed by the government, endorsed the rebuilding in early February 2025, 14 months after the disaster, sparking from Sikkim's local parties and indigenous groups like the Lepchas, who highlighted risks of further displacement—over 7,000 people were already affected—and in the fragile Himalayan ecosystem. Downstream, the Teesta-V sustained damage from a subsequent , with the Sikkim Department of Mines and conducting site investigations to recommend restoration, though full recovery timelines remain unspecified. Broader efforts include ongoing of 29 projects in Sikkim's Teesta stretch, adding to 20 already operational, amid criticism that such proliferation contributed to the flood's amplified impacts by altering river dynamics. Parallel to physical reconstruction, water allocation negotiations between and persist without resolution, hindered by upstream diversions and the 2023 flood's exacerbation of downstream scarcity in 's northern regions. Talks on a Teesta , stalled since a 2011 draft allocating 42.5% to and 37.5% to (with the rest unallocated), saw no bilateral progress in 2024-2025 following 's political upheaval. Instead, 's interim government advanced a China-funded Teesta River Comprehensive Management and Restoration Project, valued at over $1 billion, aimed at , embankment reinforcement, and expansion, which ignited widespread protests in October 2025 demanding enforcement of prior - understandings over foreign intervention. Indian officials view the Chinese initiative as a strategic maneuver to circumvent negotiations, potentially undermining 's leverage via upstream barrages like Gajoldoba, while attributes chronic dry-season shortages—reducing river flow to 1,000 cusecs against a needed 5,000—to Indian abstractions supporting West Bengal's agriculture. No new meetings have yielded concessions as of October 2025, with the dispute risking escalation amid competing geopolitical influences.

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