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Manas River
Manas River on the Assam-Bhutan border.
Manas River drainage basin
Manas River is located in Assam
Manas River
Mouth of Manas river in Assam, India
Manas River is located in India
Manas River
Manas River (India)
Location
CountryIndia, Bhutan, China
Physical characteristics
Length400 kilometres (250 mi)
Basin features
River systemBrahmaputra River

The Manas River (pron: [ˈmʌnəs]), known in Bhutan as the Drangme Chhu, is a transboundary river in the Himalayan foothills between southern Bhutan, India, and China. It is the largest of Bhutan's four major river systems,[1] with the other three being the Amo Chu or Torsa River, the Wang Chu or Raidak, and the Puna Tshang Chu or Sankosh. It is met by three other major streams before it again debouches into India in western Assam. The river flows for a total length of 400 kilometres (250 mi), including through Tibet for 24 km (15 mi), Bhutan for 272 kilometres (169 mi), and Assam for 104 kilometres (65 mi) before it joins the mighty Brahmaputra River at Jogighopa. A major tributary of the Manas, the Aie River, joins it in Assam at Bangpari.[2][3]

The Manas river valley has two major forest reserves, namely Royal Manas National Park (43,854 hectares (108,370 acres), established in 1966) in Bhutan and the contiguous Manas Wildlife Sanctuary (established at 391,000 hectares (970,000 acres) in 1955 and increased to 95,000 hectares (230,000 acres) in December 1985), encompassing a Project Tiger reserve, an elephant reserve and a biosphere reserve, which constitutes a UNESCO World Heritage Site declared in December 1985.[4][5]

Geography

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The Manas River drains 41,350 square kilometres (15,970 sq mi) of eastern Bhutan and northeast India. It has three major branches: the Drangme Chhu, Mangde Chhu, and Bumthang (Chamkhar) Chhu that cover most of eastern Bhutan, with the Trongsa and Bumthang valley also forming part of its catchment. The area drained in Bhutan territory is 18,300 km2 and is bounded by the geographical coordinates 26°13′01″N 90°37′59″E / 26.217°N 90.633°E / 26.217; 90.633.[6][7] A part of the main stem of the river rises in southern Tibet at an altitude of 4,500 metres (14,800 ft) before entering India in the Tawang district at the northwestern corner of Arunachal Pradesh.[8]

The river flows through V-shaped gorges in a southwesterly direction between two ranges of the Lower Himalayas in Bhutan and enters Assam in the south-central foothills of the Himalayas. The valley widens in the foothills, where it is marked by the formation of swamps and marshes in the river's alluvial plain. The upper catchment is largely snowbound while the middle and lower catchment are thickly forested.[8]

Clear waters of the Manas River in the sanctuary

The total combined length of all tributaries of the river system in Bhutan amounts to 3,200 kilometres (2,000 mi). The main stem of the river, the Tawang Chu, originates in the Tawang district of Arunachal Pradesh in India and after flowing in a southwesterly direction enters Bhutan near Trashigang. At Duksum in Trashi Yangtse, the Tawang Chu is joined by the Kholong Chu, which rises in the northern Himalaya snow ranges of Bhutan. Further downstream, the Gamri Chu, flowing down from Sakten, joins it. At Trashigang, the width of the riverbed is about 550 metres (1,800 ft) and the riverbed elevation is 606 metres (1,988 ft).

The Lhobrak, or Kuri Chhu, is the main central tributary of the Manas. It is the only river that rises north of the Great Himalayas and it joins the Manas at Gongri Zomsa between Monggar and Pema Gatshel districts. Further downstream, at Panbang in Zhemgang District, the combined stream joins with the Mangde Chhu. Here the riverbed elevation is 121 metres (397 ft).[2] The Mangde Chu is formed by two rivers, namely the Mangde Chu, which rises in northern Bhutan near Kula Kangri Peak where the elevation is 5,666 metres (18,589 ft), and the Bumthang River, also called the Chamkhar Chu.[2]

After flowing in a generally southwesterly direction for about 29 kilometres (18 mi) in Bhutan, the Aie River, also known as the Mow River in Sarpang District in Bhutan, joins the Manas in the Goalpara District of Assam at the village of Agrong. From here, it follows a meandering course for about 75 kilometres (47 mi) and empties into the Brahmaputra near Jogighopa. The Aie River, which rises in the Black Mountains at an altitude of about 4,915 metres (16,125 ft) near the village of Bangpari, is about 110 kilometres (68 mi) in length. The total length of the Manas, as measured along its longest tributary the Kur, is 376 kilometres (234 mi), of which about 104 kilometres (65 mi) lies in India.[3] About 270 kilometres (170 mi) of this length are in hilly or mountainous topography, with the balance in the plains.[5] The Manas River is the largest north bank tributary of the Brahmaputra, with a recorded maximum discharge of 7,641 cubic metres, and contributes 5.48% of the total average volume of the Brahmaputra.

The river valley in the foothills is surrounded by small meadows located among thickly deciduous forested foothills with many rivulets, streams and natural drainage channels related to the river system.[9] In the lower reaches of the river, there are many smooth sandy stretches populated with trees.[10] The lowlands north of the confluence with the Brahmaputra, known as the Terai or Duars (a Sanskrit word meaning "passes" or "gates"), encompass a 15–30 kilometres (9.3–18.6 mi) stretch of the outermost foothills and are very fertile. Each Duar (with elevation varying from 100 metres (330 ft) to near sea level as they join the Brahmaputra) is named after a stream or hemmed between two streams. The fertile lands have been developed into tea estates and paddy fields.[11]

Hydrology

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The Manas catchment is almost wholly mountainous, rising within the space of 140 kilometres (87 mi) from an elevation of about 100 metres (330 ft) near the Indian border to the great Himalayan peaks at over 7,500 metres (24,600 ft) along the main Himalayan range bordering Bhutan and Tibet. The huge elevation range and varied climatic conditions are reflected in rich diversity of fauna and flora native to the area.[5]

Climate

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Climate along the river is extremely varied, ranging from hot, humid subtropical conditions in the south to cold, dry alpine conditions in the north. From May to October, the southwest monsoon brings heavy rainfall—more than 4,000 millimetres (160 in) in the southern part—and there is a pronounced dry season in winter. Further north, the rainfall is generally scanty, of the order of 600 to 700 millimetres (24 to 28 in) recorded from June to August.[5] The difference between maximum and minimum river flow in the monsoon season and dry season is said to be as much as 20 times.[5]

Protected areas

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A suspension bridge across the Manas River in a reserved area of the park

Out of the large catchment of the river valley, many protected areas or reserves have been specifically demarcated, both in Bhutan and India, which are declared national parks or sanctuaries. The two forest and wildlife reserves cover an area of 9,938.54 square kilometres (3,837.29 sq mi), about 24% of the total catchment area of 41,350 square kilometres (15,970 sq mi) of the Manas basin.[citation needed]

Royal Manas National Park

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Main articles: Manas National Park and Royal Manas National Park

Royal Manas National Park in southern Bhutan, considered the national heritage of Bhutan, was first declared a wildlife sanctuary and subsequently raised to the status of a national park in 1993.[citation needed] Covering an area of 9,938.54 square kilometres (3,837.29 sq mi), the area is 92% forested and is a well-preserved example of a natural Eastern Himalayan ecosystem.[citation needed] The park is bordered to the north by Jigme Singye Wangchuck National Park and to the south by the Manas Tiger Reserve in India. Within Bhutan, biological corridors linked with the park include Phrumsengla National Park in the north, Phibsoo Wildlife Sanctuary in the west, and Jomotsangkha Wildlife Sanctuary in the east.[citation needed] The park features a large diversity of tropical grasslands, moist temperate forests, alpine meadows, and scrublands.[citation needed] The various flora and fauna identified in the park are 45 species of mammals, 366 species of birds, and 900 species of vascular plants. The World Wildlife Fund (WWF) is actively working on a conservation management plan in association with local wildlife authorities to preserve and protect this national heritage.[12] Among the important faunal species living here are the royal Bengal tiger (Panthera tigris tigris), Asian elephant (Elephas maximus), gaur (Bos gaurus), four rare species of golden langur (Trachypithecus geei), pygmy hog (Sus salvanius), hispid hare (Caprolagus hispidus), greater one-horned rhinoceros (Rhinoceros unicornis), and wild Asiatic water buffalo (Bubalus arnee). There are 362 species of birds, out of which four species of hornbills (rufous-necked, wreathed, pied, and great Indian) have been recorded. Apart from the Ganges river dolphin (Platanista gangetica) other aquatic species identified in the Manas river are the deep-bodied mahseer (Tor tor), golden mahseer (Tor putitora), and chocolate mahseer or katle (Acrossocheilus hexangonolepis). Five thousand people live within the park limits in several villages.[13] Tigers, the most revered animal in Bhutan, are estimated to number about 100 and are mostly confined to this national park and the neighbouring Manas National Park in India.[14]

The world's rarest monkeys, the golden langur, which flourish in dense forests with its long tail with a tassel at the end, are found in both Bhutan and India, in the two reserved forest sanctuaries. These monkeys, found in groups, have no hair on their black face but have generous golden ruff on their body. They are found in large numbers – approximately 180 in India and 1,200 in Bhutan, as per counts made in 1978 and 1980 respectively.[15]

Manas Wildlife Sanctuary

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Main article: Manas Wildlife Sanctuary
Manas Wildlife Sanctuary in the Manas valley

The Manas Wildlife Sanctuary, located in Assam, is considered one of "Asia's finest wild life reserves" and is a UNESCO World Heritage Site. It is contiguous with Bhutan's Royal Manas National Park to its north. It now encompasses a bioreserve, a tiger reserve, and also an elephant reserve. The park is well forested and also comprises grasslands and marshes. In 1928, the core of the area was designated a sanctuary and in 1978 it was declared a tiger reserve.[16]

The Manas River and its tributary the Hakua flow through the sanctuary. A thick mantle of alluvium is the dominant soil of the terrain. Under subtropical climatic conditions (with 30 millimetres (1.2 in) of annual rainfall and temperature varying between a maximum of 30 °C (86 °F) and a minimum of 5 °C (41 °F)), the forest consists of the semi-evergreen forest vegetation with mixed deciduous, littorals, and swamps, and interspersed with bamboo and cane. Flooding occurs in large parts of the bioreserve.[17][18]

The park is managed under several conservation management units such as the Core Zone, the Buffer Zone, and the Economic Zone. The park is known for its rare and endangered endemic wildlife such as tigers and elephants, as well as the Assam roofed turtle, hispid hare, golden and capped langurs, pygmy hog, one-horned rhinoceros, Asiatic buffalo, swamp deer, barking deer, leopard, clouded leopard, marbled cat, sloth bear, hoolock gibbon, wild boar, and river dolphin. The park is home to 22 endangered mammal species. Reptiles include pythons, common Indian crocodiles, gharials, common wolf snakes, cat snakes, and many other species. Birds include hornbills, common cranes, common redshanks, Eurasian woodcocks, spotted eagles, black-throated divers, little grebes, various types of herons, black ibises, Eurasian sparrowhawks, spot-bellied eagle-owls, and several others. Fish species include katli, jurraha, chenga, telliah, labeo, and mahseer.[19] Conservation measures have been undertaken to prevent poaching, overfishing, encroachments, and many other related issues. These measures are meant to minimise human interference in the fragile core zone, to enable the creation of a database, and to carry out research on animal and plant populations for better conservation of the ecosystem.[20]

In 1980, the park was central to the Bodoland agitation of Assam as the Bodos dominating the area took refuge in the sanctuary. Bodos have been demanding autonomy or a separate state of their own on grounds that their lands were incorporated into Assam during the British Raj.[19]

Environmental issues

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A Brahma temple in the Manas valley at Kokrajhar in Assam

The Manas River has often been at the centre of environmental controversies, particularly in the 1980s. Two dams were proposed on the Bhutanese side of the river to provide hydroelectric power and to control the flow of the Brahmaputra on its northern bank and to make way for irrigation schemes. However, there were not only local concerns but national and international ones among environmentalists with regard to the proposals, who lobbied enough support to ensure that the dam proposals were dropped in 1986.[21] In February 1989, the All Bodo Students Union (ABSU) invaded the park and killed several wardens and guards, permitting the entry of poachers and loggers who posed an immediate threat to the wildlife of the park and its river. The threat of flooding remained as ever in 2010.[21]

One of the development projects planned in the past on the Manas River envisaged flood control in the Brahmaputra River and augmentation of flows in the Ganga river system by building a dam on the Manas at the Indo-Bhutan border. The water stored behind the reservoir was proposed to be transferred through a long canal system through the foothills of the Himalayas (skirting Bangladesh), crossing 25 major and minor rivers, out of which the major rivers the Sankosh, Raidak, Amo (Torsa), Karatoya, Teesta, Atrai and Mahananda flow through North Bengal, and finally into the Kosi River in North Bihar. The project has not proceeded further due to adverse public opinion and environmental concerns.[22][21]

A proposal mooted in the 1970s to build a dam on the river for multipurpose uses of power, irrigation, and flood control in Assam involved a 100-kilometre-long (62 mi) canal from the Manas reservoir to another reservoir on the Sankosh River. As the canal would have passed through the Manas Tiger Reserve, the Ministry of Environment and Forests of the Government of India objected to the proposal on the grounds of adverse impacts on the hydrology and ecology of the area that would occur due to the dam. This view was also supported by the World Heritage Committee of UNESCO. The late Prime Minister of India Rajiv Gandhi upheld the objections and decided to discontinue the project. It is unlikely to be revived.[23] The proposal had been mooted as a joint project of India and Bhutan. The pre-feasibility report prepared for this Manas multipurpose project envisaged power generation of 2800 MW. Another cooperation project on the Mangde Chu, a tributary of the Manas in central Bhutan, envisages power generation of 360/600 MW, for which a Detailed Project Report (DPR) is under preparation.[24]

See also

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Wikimedia Commons has media related to Manas River.

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Manas River

View on Grokipedia
from Grokipedia
The Manas River, known as the Drangme Chhu in , is a in the foothills that originates near the -Tibet border and flows southward through and into India's state, where it joins the as a major tributary. With a total length of 376 kilometers—272 kilometers traversing and the rest through —the river's course is shaped by heavy rainfall, leading to dynamic sediment deposition and frequent flooding that influence the surrounding alluvial plains and forests. It demarcates sections of the -India border and sustains the Manas Wildlife Sanctuary, a encompassing 39,100 hectares of diverse habitats critical for including the , , and greater one-horned , underscoring its role in transboundary conservation efforts between the two nations.

Etymology and Historical Context

Name and Cultural Significance

The name of the Manas River derives from , a serpent goddess in associated with snakes, poison, and protection from venomous bites. In regional folklore, Manasa is depicted as a powerful whose worship involves rituals to avert snakebites, reflecting the river's location in snake-prone tropical forests of and . One account links the river's nomenclature to Manasa as the daughter of the sage , emphasizing her mythological origins in ancient where she emerges as a folk challenging higher gods for recognition. In , the is known as Drangme Chhu, a term from the local language denoting a , highlighting its swift, turbulent flow rather than mythological ties. Culturally, the Manas holds significance among indigenous communities in the region, such as the Bodo and other Assamese tribes, where it features in oral traditions and local deities' lore, often invoked for fertility and protection amid seasonal floods. These narratives underscore the river's role in sustaining riparian ecosystems that support traditional livelihoods, though primary reverence stems from Hindu folk practices centered on rather than broader Vedic or Buddhist cosmologies.

Pre-Modern Human Interactions

The Bodo people, belonging to the broader , represent one of the earliest documented human populations in the Manas River valley on the Indian side, with scholarly estimates placing their migration into between approximately 2000 BCE and 1000 BCE based on linguistic and oral historical analyses. These indigenous communities established semi-permanent settlements along the river's fertile alluvial plains, leveraging its seasonal floods for silt deposition that supported wet-rice cultivation, fishing with traditional traps and nets, and rearing of livestock such as pigs and cattle integral to their animistic Bathou faith. Archaeological inferences from regional burial sites and megalithic structures in western suggest continuity of such riparian lifestyles from the late prehistoric period through the early medieval era, though direct excavations in the immediate Manas floodplain remain limited. In the Bhutanese upper reaches, known locally as Drangme Chhu, pre-modern interactions centered on small-scale clan-based societies of Tibeto-Burman origin, including proto-Monpa and Tshangla groups, who utilized the river for seasonal herding of yaks and extraction of timber, , and riverine resources like fish and alluvial prior to Bhutan's unification under the Drukpa lineage in the . The river's course facilitated intra-regional mobility, with oral traditions recounting its role in connecting foothill villages to higher pastures during monsoonal retreats, while serving as a defensive against lowland incursions. Medieval political dynamics further shaped interactions, as the Manas demarcated territories between emerging Assamese kingdoms—such as the 16th-century Koch dynasty, which extended influence up to the river's eastern banks—and Bhutanese dzongponates administering the southern duars. This boundary function enabled limited barter trade in commodities like Bhutanese pods, salt, and exchanged for Assamese and iron tools, though records indicate sporadic conflicts over control of riverine crossing points rather than sustained . Indigenous groups on both sides maintained animistic reverence for the river, attributing floods and bounty to local deities, a practice persisting into the before broader Hindu and Buddhist influences.

Physical Geography

Course and Length

The Manas River, also known as Drangme Chhu in , originates in the glacial zones of the in southern , , flowing eastward for approximately 24 kilometers before crossing into . In , it courses southward through rugged mountainous terrain, covering 272 kilometers and draining significant portions of the country's southern regions, including areas near Panbang town. This segment features steep gradients, gorges, and forested valleys characteristic of the Himalayan foothills. Entering in the northeastern state of , the river travels an additional 104 kilometers eastward, partially forming the international boundary with along its lower reaches. It meanders through the alluvial plains of the , supporting diverse riparian ecosystems, before merging with the near Jogighopa in Assam's . The total length of the Manas River is approximately 400 kilometers, encompassing its transboundary path across , , and .

Basin and Tributaries

The Manas River basin covers approximately 37,500 square kilometers, primarily within central and eastern , with extensions into , . This transboundary catchment drains a significant portion of Bhutan's territory, encompassing diverse terrain from high glaciers to lowland floodplains. In Bhutan, the basin is delimited by the northern Bhutanese Himalayas, while in India, it is bounded by the Pohumara River basin to the east, the Champamati River basin to the west, and the Brahmaputra floodplain to the south. The basin is structured around four major sub-basins in : Mangde Chhu, Chamkhar Chhu, Kuri Chhu, and Dangme Chhu, which collectively feed the main stem of the Manas. The Kuri Chhu stands out as a key , originating north of the Great Himalayan range and joining the Manas after traversing rugged terrain. The Mangde Chhu sub-basin includes catchments like Bjizam (1,390 square kilometers) and Tingtibi (3,339 square kilometers), while the Dangme Chhu features extensive areas such as Panbang (21,006 square kilometers). In the Indian portion, the Manas receives additional tributaries including the Aie River, which joins near Bangpari in Assam, enhancing the river's discharge before its confluence with the Brahmaputra near Jogighopa. The eastern branch of the upper Manas is known as the Beki River, contributing to the overall hydrological network. These tributaries sustain high sediment loads and seasonal flooding characteristic of the basin's monsoonal dynamics.

Hydrology

Discharge and Flow Patterns

The Manas River exhibits a highly variable discharge regime dominated by the seasonal monsoon cycle, with peak flows driven primarily by intense rainfall in its Himalayan catchment spanning Bhutan and northern India. Average annual discharge is approximately 1,100 cubic meters per second (m³/s), representing about 5.5% of the Brahmaputra River's total inflow from northern tributaries. This estimate derives from the river's proportional contribution to the Brahmaputra's mean discharge of around 20,000 m³/s, as measured at downstream gauging stations. Flows are sustained by a combination of monsoon precipitation, which accounts for the majority of annual runoff, and contributions from glacial and snowmelt in the upper Bhutanese reaches, though the latter provide more stable baseflow during non-monsoon periods. Discharge peaks sharply during the summer from June to September, when heavy orographic rainfall—often exceeding 2,000 mm in the Bhutan foothills—triggers widespread flooding across the plains. Recorded maximum discharges have reached 7,641 m³/s during extreme events, leading to channel overflow and sediment-laden floods that reshape the braided lower course. Monthly peak flows typically occur in or , comprising up to 70-80% of annual discharge volume, with rapid rises following storm events due to the steep gradient from the (over 3,000 m drop in the first 200 km). Variability is amplified by the transboundary nature of the basin, where synchronized heavy rains in and synchronize high runoff timing. In the from to , discharges drop to minimal levels, often below 200 m³/s, with the river narrowing and exposing beds in shallower sections, approaching near-dry conditions in tributaries. This low-flow phase relies on residual and seepage, resulting in clear, low-sediment water characteristic of pre-monsoon stability. Interannual fluctuations are influenced by El Niño-Southern Oscillation patterns, with weaker monsoons linked to 10-20% reductions in peak flows, as observed in hydrological records from gauging sites spanning 1999-2019. Such patterns underscore the river's sensitivity to regional climate dynamics, with no long-term trend toward increased or decreased variability evident in available data up to 2020.

Sediment Transport

The Manas River transports substantial volumes of , primarily derived from in its steep Himalayan catchment within , where tectonic activity, glacial melt, and intense monsoonal runoff mobilize material from and . Suspended load dominates during high flows, with coarser bedload contributing to channel bed , while the overall high supply exceeds the river's transport capacity in the lower alluvial plains of , fostering braided morphology with multiple anastomosing channels and mid-channel bars. This pattern is evident in the reaches, where the river's association with high gradients and heavy influx promotes dynamic reconfiguration of the channel belt. Seasonal variability drives flux, with over 90% occurring during the June-to-September , when peak discharges amplify and suspension of fines, leading to turbid flows and downstream deposition. events exacerbate this, as large discharges coupled with elevated loads induce lateral migration and undercutting, with the river's instability documented through geospatial analyses showing consistent channel shifts in upper . Such dynamics not only reshape local but also contribute to the Brahmaputra's prodigious basin-wide budget, influencing delta progradation and processes. Anthropogenic factors, including upstream and land-use changes, may intensify yields, while climatic shifts in could modulate transport efficiency, potentially reducing peak loads but altering depositional patterns and ecological habitats. Precise quantification of annual yield remains constrained by sparse gauging, though the river's role in overloading the system aligns with broader Himalayan foreland patterns of tectonic-driven .

Climate Influences

Monsoonal Regime

The Manas River exhibits a classic monsoonal hydrological regime typical of Himalayan foothill rivers, where discharge is highly seasonal and dominated by precipitation from the South Asian summer monsoon. Over 75% of the basin's annual rainfall, averaging 2,000–4,000 mm in upstream areas, occurs between June and September, driven by moisture-laden southwest winds orographically enhanced by the Bhutan Himalayas. This results in rapid runoff generation across the steep, forested catchment, with peak discharges concentrating 70–80% of the mean annual flow in these months, often leading to inundation of riparian zones and downstream Assam floodplains. In contrast, the non-monsoon period from to May features markedly reduced flows, sustained primarily by and episodic snowmelt from higher elevations in , where winter contributions from northern circulations play a minor role. River levels can drop to near-dry conditions in smaller tributaries by November–February, reflecting the basin's reliance on recharge for sustenance. This bimodal pattern underscores the river's vulnerability to intra-annual variability, with erratic onset or intensity—such as prolonged dry spells or excessive downpours—directly altering flow peaks and risks. Orographic lifting amplifies monsoon precipitation gradients, with upstream Bhutanese sections receiving heavier totals than downstream Indian reaches, contributing to high sediment-laden floods that reshape channel morphology annually. Historical analyses from 2005–2018 indicate fluctuating pre-, intra-, and post-monsoon rainfall trends, influencing discharge timing and magnitude without overriding the dominant seasonal pulse. The Manas River displays pronounced seasonal variability in discharge, with approximately 75% of its annual flow occurring during the summer period from May to , when intense rainfall in the catchment—averaging 3,300 mm annually—drives peak discharges exceeding 10,000 m³/s at gauging stations in . In contrast, dry season flows from November to February diminish significantly, often resulting in low water levels or partial drying in tributaries due to minimal precipitation and reliance on from rainfall rather than substantial glacial or contributions (limited to 2-12% of total discharge). This bimodal pattern amplifies flood risks during monsoons and in winters, influenced by the river's transboundary Himalayan origins and monsoon-dominated regime. Long-term hydrological trends in the basin reflect climate-driven shifts, including a gradual decline in rainfall frequency from 2005 to 2019 across annual and monsoon seasons, potentially linked to altered precipitation patterns under global warming. Flood characteristics show upward trajectories, with modeled peak discharges for the Manas increasing alongside extended flood durations (from medians of 15.2-15.7 days baseline to 19.3-20.1 days projected) and rising water levels (approximately 1.45 m higher under frequency distributions like Gumbel). Projections from hydrological models (HEC-HMS) under RCP4.5 and RCP8.5 scenarios indicate annual flow increases of 6% and 15% by 2070-2100, respectively, with summer monthly flows rising 14-43% (e.g., +18-29% in May-August) while winter flows show minor declines, exacerbating intra-annual variability through wetter monsoons and drier winters. These trends heighten flood wave frequency in pre-monsoon periods and overall peak flows (likely increases of 7-28% at key Brahmaputra confluences), posing risks to downstream ecosystems and infrastructure without adaptive measures like reservoirs.

Ecology and Biodiversity

Habitat Types

The Manas River traverses a mosaic of habitats influenced by its Himalayan origins, seasonal flooding, and alluvial deposition, fostering distinct ecological zones from Bhutanese foothills to Indian floodplains. These include riverine tracts along shifting channels, alluvial grasslands in the Terai lowlands, marshlands within the riverine wetlands, and Bhabar savannahs on piedmont terraces with coarse, permeable soils. Forest cover adjacent to these aquatic and grassland systems comprises semi-evergreen formations in northern uplands and mixed moist-dry deciduous stands in successional areas renewed by floods. Alluvial grasslands dominate the southern plains, featuring fertile, seasonally inundated savannahs with tall grasses such as and Phragmites karka, which support high densities and act as fire-prone buffers against forest encroachment. Riverine habitats, marked by dynamic braiding and , host riparian vegetation including thickets and on fresh silt deposits, while wetlands and oxbow lakes provide perennial water refugia amid monsoonal variability. In the Bhutanese upper basin, subtropical broadleaf forests transition to tropical types below 500 meters, interspersed with grassland patches and broad riverbeds that deposit nutrient-rich . Vegetation diversity reflects habitat heterogeneity, with over 89 tree species (e.g., sal in deciduous zones), 172 herbs, and 43 grasses documented across these types, sustained by the river's and hydrological regime. savannahs, with sandy loams and , grade into tracts of finer and shallow aquifers, enabling wetter understories that enhance overall plant richness. These habitats collectively underpin the river's role as a biodiversity corridor, though flood-induced shifts can alter boundaries between grasslands and encroaching forests.

Key Species and Endemism

The Manas River ecosystem supports a diverse array of , particularly in its grasslands, riverine forests, and wetlands, which serve as critical habitats for large mammals and aquatic species. Key mammalian species include the (Panthera tigris tigris), (Elephas maximus), greater one-horned rhinoceros (Rhinoceros unicornis), and (Bubalus arnee), all of which rely on the river's seasonal flooding for foraging and migration corridors. The river also harbors aquatic reptiles such as the (Pangshura sylhetensis), which inhabits its slower-flowing sections and tributaries. Avian species of note encompass the endangered (Houbaropsis bengalensis), a grassland bustard dependent on the river's alluvial plains for breeding. Endemism in the Manas River basin is pronounced among and specialists, reflecting the unique biogeographic isolation of the eastern Himalayan foothills. The (Porcula salvania), critically endangered and restricted to tall riverine , represents one of the world's rarest mammals, with conservation efforts including reintroductions into Manas habitats as of 2024. The (Caprolagus hispidus), another endemic vulnerable to habitat loss from flooding and encroachment, persists in the river's grassy floodplains. Similarly, the golden langur (Trachypithecus geei), endemic to the Indo-Bhutan forests along the Manas corridor, occupies semi-evergreen riverine patches, with populations bolstered by transboundary protections. These species underscore the river's role as a refuge, though ongoing threats like and hydrological alterations challenge their persistence.

Protected Areas and Conservation Zones

Royal Manas National Park

Royal Manas National Park covers 1,057.28 square kilometers in southern Bhutan, making it the kingdom's oldest protected area, initially designated as a game sanctuary and elevated to national park status in 1993. The park adjoins India's Manas Tiger Reserve, creating a contiguous transboundary conservation zone that facilitates wildlife movement across the international border. The Manas River traverses and partially bounds the park, delivering seasonal floods that maintain alluvial grasslands and riparian forests essential for herbivore grazing and predator habitats. Habitat diversity within the park includes 1,191.18 hectares of grasslands, tropical monsoon forests, subtropical broadleaf forests, and wetlands, all shaped by the river's sediment deposition and hydrological regime. These river-influenced ecosystems support a rich array of species, including the endangered , with 29 adults documented in a January 2025 camera-trap survey representing over 22% of Bhutan's total population of 131 individuals. Additional flagship species encompass the golden langur, , hispid hare, and , many of which rely on riverine corridors for dispersal and foraging. Conservation efforts emphasize anti-poaching patrols, habitat restoration, and community-based , bolstered by the Bhutan for Life initiative, which secures long-term financing for management until 2047. The park's inclusion on UNESCO's Tentative World Heritage List underscores its global biodiversity significance, particularly in preserving Eastern Himalayan endemism amid river-driven ecological processes. Transboundary cooperation with addresses shared threats like poaching and , enhancing the Manas River basin's overall resilience.

Manas Tiger Reserve

The Manas Tiger Reserve is a protected area in , , designated under in 1973 with the Manas Wildlife Sanctuary as its core zone. Originally established as a wildlife sanctuary on October 1, 1928, covering 391 square kilometers, it expanded into a tiger reserve encompassing 2,837 square kilometers to conserve tigers and associated . The reserve lies along the Manas River, which demarcates the -Bhutan border, integrating floodplain grasslands, subtropical forests, and riverine habitats critical for wildlife movement. In 1985, the core area was inscribed as a for its outstanding universal value in representing Eastern Himalayan hotspots. The reserve's tiger population plummeted during the late 1980s to early 2000s due to intensified amid Bodo and ethnic conflicts in the Bodoland , reducing numbers to fewer than 10 individuals by 2008 and prompting its listing on UNESCO's World Heritage in Danger roster from 1992 to 2011. Post-2003 peace accords, rigorous anti- patrols, habitat restoration, and community engagement reversed the decline, with camera-trap surveys documenting a tripling of tigers to over 30 adults by 2022, achieving an annual growth rate of 17 percent through 2025. This recovery reflects effective enforcement by the Forest Department and collaborations with organizations like , including SMART (Spatial Monitoring and Reporting Tool) patrols that reduced incidents by over 90 percent since 2010. Beyond tigers, the reserve supports viable populations of greater one-horned rhinoceros (over 30 individuals as of 2023), Asian , and like the and swamp deer, sustained by the river's seasonal flooding that maintains alluvial grasslands. Transboundary cooperation with Bhutan's enhances connectivity, allowing genetic exchange and migration across 1,000 kilometers of shared landscape, formalized through bilateral agreements since 2010. Management plans emphasize control, , and eco-development for fringe communities to mitigate human-wildlife conflicts, such as crop raiding by . Persistent challenges include occasional retaliatory , , and encroachment, exacerbated by upstream in and climate-induced variability along the Manas River. Despite these, the reserve's delisting from UNESCO's danger list in 2011 underscores successful interventions, with ongoing monitoring via satellite telemetry and community-based conservation yielding sustained prey base recovery, including hog deer and herds essential for prey dynamics.

Human Uses and Economic Role

Irrigation and Agriculture

The Manas River's floodplains in , , deposit nutrient-rich that bolsters soil fertility for in adjacent districts including Baksa, Chirang, , and , where paddy dominates cropping patterns alongside , pulses, and . These alluvial areas, formed by seasonal inundation from the river and its tributaries like the Aie and Pagladia, support subsistence farming for fringe communities reliant on single-crop rice systems yielding approximately 2-3 tons per hectare under rainfed conditions. However, direct infrastructure remains minimal within the protected Manas Reserve, with confined to buffer zones vulnerable to crop raiding by wildlife such as . Minor schemes, managed by Assam's Department, extract water from the Manas and tributaries like the Burisuti for supplemental use during dry periods, covering small command areas of under 100 hectares per scheme in districts like . These systems, often community-operated, prioritize paddy transplantation and horticultural crops, though they constitute less than 10% of cultivable land in the basin due to flood risks and conservation restrictions. In , upstream portions of the basin utilize second- and third-order tributaries of the Manas (known as Drangme Chhu) for community-managed schemes, irrigating around 64,000 acres nationwide as of recent assessments, with southern districts like Sarpang and drawing perennial flows for terraced paddy fields yielding up to 1.93 metric tons per acre post-improvement. Proposed diversions under the Manas-Sankosh-Teesta-Ganga interlinking project aim to transfer surplus discharge—estimated at 43 billion cubic meters annually from the Manas and nearby rivers—to irrigate 372,000 hectares across , , and , potentially stabilizing dry-season farming and mitigating deficits in Ganga-dependent areas. Ongoing initiatives like the Integrated River Basin Project focus on flood control to protect existing agricultural lands rather than expansion, reflecting tensions between conservation priorities and livelihood needs in the transboundary basin.

Fisheries and Local Livelihoods

The fisheries of the Manas River sustain local communities primarily along its course in , , where constitutes the main occupation and source of for riverside residents. Fishermen utilize a variety of traditional crafts, including dugout canoes and planked boats, alongside gears such as cast nets, drift gill nets, and hook-and-line setups to target migratory and resident populations. These practices exploit the river's seasonal abundance, particularly during post-monsoon periods when water levels recede and congregate in shallower reaches. Prominent among exploited species is the golden mahseer (), an endangered large cyprinid capable of reaching lengths up to 2.7 meters and weights exceeding 50 kilograms, valued for its sporting qualities and ecological role in the . The Manas supports diverse ichthyofauna, with surveys in the adjacent documenting at least 35 confirmed species across multiple families, including cyprinids, bagrids, and sisorids, alongside uncertain records for nine additional taxa. In Bhutanese sections, the golden mahseer holds cultural reverence as one of eight auspicious symbols in , influencing conservation measures that restrict local harvesting to promote population recovery. Local livelihoods face tensions between subsistence needs and regulations, as communities in border villages rely on riverine protein and income amid limited alternatives. Initiatives in emphasize alternative employment to curb illegal fishing, recognizing the mahseer's role in maintaining while supporting human dependence. On the Indian side, unregulated capture contributes to pressure on , though the river's transboundary flow enables collaborative monitoring to sustain yields for approximately 1,000-2,000 fisher households in Assam's Manas-adjacent districts.

Flooding and Hazard Dynamics

Causes and Frequency

Flooding in the Manas River is primarily driven by intense , which delivers the bulk of annual rainfall—often exceeding 165 cm in the plains—over a concentrated four-month period from to , overwhelming the river's capacity due to its Himalayan origins and steep topographic gradients that accelerate runoff. High sediment loads from erosion in the upstream Bhutanese cause riverbed , channel braiding, and frequent shifts in course, amplifying propagation into the Brahmaputra floodplain. Secondary anthropogenic factors include upstream hydroelectric dams in , such as the Mangdechu project, which release stored water abruptly, triggering s; for instance, the 2016 event from Mangdechu inundated 1,000 hectares of cropland and displaced 30,000 people downstream in Manas areas due to the tributary's high (853 km/sq km) and bifurcation ratio indicating inherent proneness. A December 2024 , the first major event since 1985, was attributed to a under construction near the Bhutan-India border, breaching roads and destroying infrastructure in . Floods recur annually during the , with peak discharges typically in and when rainfall intensity maximizes, as observed in Brahmaputra basin tributaries including Manas. Hydrological modeling based on from the 1990s to 2020s fits the for , projecting peak levels of 46.62 m (5-year ) to 48.07 m (200-year ), corresponding to inundated areas expanding from 493.54 km² to 673.72 km². Notable historical peaks include events in 1962 and 1988 that severely impacted Assam's Manas-adjacent regions.

Historical Events and Impacts

The Manas River, prone to seasonal flooding due to heavy rains and upstream glacial melt from the , has experienced several major inundation events that have significantly impacted the surrounding ecosystems and human settlements in , . Historical records indicate recurrent floods exacerbating erosion and sediment deposition, altering and habitats. In the Manas-Beki river system, flooding typically peaks during to , with rapid water level rises causing short but intense inundations on sloping terrain. A notable early event occurred in 1985, when flash floods eroded sections of the , marking one of the last major pre-dam era incidents before subsequent infrastructure developments upstream. This flood washed away riverine grasslands and displaced wildlife, though specific casualty figures remain undocumented in available hydrological data. Subsequent major Assam-wide floods in 1988 devastated large tracts of the Brahmaputra basin, including Manas areas, leading to widespread and agricultural losses estimated in millions of rupees, with riverbank shifts burying settlements under . The 1988 event, driven by excessive precipitation exceeding 300 cm annually in the catchment, highlighted the river's braided channel dynamics, which amplify flood propagation downstream. In the , the 1998 flood stands out, affecting higher average areas than 1980s events and inundating over 40% more in the , including Manas River stretches. This resulted in crop destruction across thousands of hectares and of riparian communities reliant on fisheries. The saw intensified flood frequency partly due to land-use changes, with average inundated areas rising compared to prior decades. By the 2000s, the 2004 flood triggered ecological shifts in , promoting swamp grassland expansion at the expense of tall grasslands, which reduced habitat suitability for species like the . Infrastructure damage included washed-out roads and bridges, hindering park access and anti-poaching efforts. The 2007 monsoon flood was attributed by India's Central Water Commission to excess releases from Bhutanese dams, though Bhutan contested this, citing natural overflow; it contributed to broader Assam inundations affecting Manas habitats. More acutely, the 2016 flash flood, originating from Bhutan's Mangdechu River basin, submerged 60% of Manas National Park, displacing over 30,000 people and causing wildlife drownings, including rhinos and elephants, while eroding park boundaries and depositing massive silt loads. Indian officials blamed sudden dam releases from structures like Kurichu, exacerbating downstream flows, whereas Bhutanese authorities maintained the event stemmed from uncontrolled monsoon runoff. This incident underscored transboundary tensions, with floodwaters destroying bridges and roads, and long-term impacts including altered river channels and biodiversity loss estimated in park core zones.

Environmental Challenges and Management

Deforestation and Encroachment

During the civil unrest period spanning the late 1980s to early 2000s, , which encompasses the Indian stretch of the Manas River basin, suffered extensive , with over 40% of its primary forest cover cleared for agricultural fields and human settlements due to weakened enforcement and influx of insurgents and displaced populations. This degradation primarily occurred in the park's core and buffer zones along the river's riparian corridors, converting biodiverse alluvial grasslands and sal-dominated forests into croplands, exacerbating and riverbank instability. Post-conflict recovery efforts included eviction drives, such as the 2016 clearance of encroached areas in the Bhuyanpara range, which temporarily reclaimed portions of the park from illegal farming and habitation. Despite these measures, encroachments have persisted, with approximately 40 square kilometers of the park remaining under illegal agricultural occupation as of 2016, driven by local communities' demands for arable land amid population pressures. The International Union for Conservation of Nature (IUCN) identified such encroachments—for farming and settlements—as a primary ongoing threat in its 2020 assessment, noting their role in fragmenting habitats critical to the riverine ecosystem. Transboundary deforestation has compounded these issues, with rampant reported in upstream Bhutanese portions of the Manas conservation landscape as of 2016, leading to increased in the river and downstream habitat loss in . UNESCO monitoring in 2023 confirmed that agricultural encroachments continue unabated in reactive zones like Bhuyanpara, undermining the park's World Heritage status despite repeated interventions, and contributing to a where up to 40% of original extent has been permanently lost to human use. These pressures highlight enforcement gaps, as buffer areas adjacent to the river remain vulnerable to organized land grabs, per government and conservation analyses.

Poaching and Insurgency Effects

The Bodo insurgency in Assam's Bodoland region, spanning from the late 1980s to the early 2000s, severely disrupted conservation efforts in , enabling widespread and degradation along the Manas River corridor. Militant groups, including the National Democratic Front of Bodoland (NDFB), used the park's remote forests as operational bases, leading to the flight of forest guards and the collapse of administrative control, which facilitated unchecked illegal activities. This period saw insurgents extract timber and resources to fund operations, exacerbating networks that targeted high-value for international markets. Poaching intensified dramatically, resulting in the near-extirpation of key species; the (Rhinoceros unicornis) population, estimated at 85–100 individuals in the , was entirely eliminated by the early due to horn trafficking across the border. Tigers (Panthera tigris) and (Elephas maximus) faced similar decimation, with tiger numbers dropping to critically low levels amid reduced prey availability from overhunting of herbivores like wild water buffalo and swamp deer. Insurgents and opportunistic poachers exploited the security vacuum, with reports of systematic slaughter for skins, tusks, and meat, further compounded by cross-border smuggling routes along the Manas River. These activities triggered cascading ecological effects, including over 40% clearance of primary forests for settlements and , which fragmented habitats critical for riverine species and increased into the Manas River, altering its hydrological dynamics. Prey predator imbalances emerged, with large herbivores like hog deer and wild pigs declining due to intensified pressure, while the loss of apex predators disrupted webs and vegetation control in grasslands. Infrastructure sabotage, such as the destruction of anti-poaching camps and patrol routes, prolonged vulnerability, with noting extensive removal and during peak conflict years in the .

Climate Change Attributions vs. Natural Cycles

Flooding and flow variability in the Manas River, a major tributary of the Brahmaputra, are predominantly driven by seasonal dynamics, with 60-70% of annual rainfall concentrated in June-September, leading to peak discharges that have historically fluctuated due to natural interannual and decadal oscillations such as El Niño-Southern Oscillation (ENSO) and influences. El Niño events have been associated with a 31% increase in pre-monsoon flood frequency in the Brahmaputra basin, while La Niña phases elevate post-monsoon risks by 37%, underscoring the dominance of these oscillatory modes in modulating extreme events over short-term records. Tree-ring reconstructions spanning seven centuries reveal that high-magnitude discharges in the Brahmaputra, including those affecting tributaries like the Manas, fall within the envelope of pre-industrial natural variability, with modern instrumental data underestimating extreme flood hazards by 24-38% when ignoring such cycles. Empirical analyses of rainfall in the Manas basin from 2005-2018 indicate variable patterns without a consistent upward trend in intensity, aligning with broader observations (1989-2018) showing high variability but no statistically significant long-term increase in total or extreme attributable to anthropogenic . Flood frequency modeling using for the lower Manas yields levels (e.g., 46.62 m for 10-year, up to 48.07 m for 100-year events) that reflect hydrological stationarity over available records, with post-1950 increases in perceived severity linked more to the 1950 Assam earthquake's morphological alterations and land-use changes than climatic shifts. While some studies project 8% higher basin-wide annual water yields under future warming scenarios due to enhanced moisture, these rely on global climate models with acknowledged uncertainties in regional attribution, where natural variability masks signals in observed discharge trends. Claims attributing Assam's recurrent floods, including those from the Manas, directly to often stem from narrative syntheses rather than formal detection-attribution frameworks, which highlight the complexity of isolating anthropogenic signals amid dominant natural forcings like cycles and teleconnections. Peer-reviewed event analyses emphasize that while warming may amplify thermodynamic potential for extremes, dynamical factors (e.g., variability) and non-climatic amplifiers (e.g., , embankment failures) explain observed dynamics without requiring unprecedented climatic departure. In the Brahmaputra context, load and runoff records (2000-2022) show fluctuations tied to anomalies rather than a secular trend, reinforcing that natural cycles, including multi-decadal oscillations, account for the bulk of variability in Manas River hazards.

Recent Developments and Recovery

Post-Conflict Wildlife Rebound

Following the 2003 Bodoland Territorial Council peace accord, which curtailed militancy in the region encompassing along the Manas River, conservation efforts intensified, enabling a marked rebound in wildlife populations previously decimated by and habitat disruption during decades of . Armed groups had occupied park areas, halting anti-poaching patrols and facilitating the near-extinction of species like tigers and rhinos by the early 2000s; post-conflict clearance of insurgents from western ranges by 2015 restored patrol efficacy. Tiger ( tigris) numbers surged, with adult density tripling from 1.06 individuals per 100 km² in 2011–2012 to 3.64 per 100 km² in 2018–2019, reaching an estimated 57 adults by 2022 and establishing Manas as a source population for the transboundary landscape shared with . This recovery, at an annual growth rate of 17%, stems from bolstered measures including increased patrols and monitoring, though leopards ( pardus) maintained consistently high densities throughout. Asian elephants (Elephas maximus) exhibited very high densities in 2025 surveys, reflecting habitat regeneration and reduced human-elephant conflict through community partnerships post-peace accord, while one-horned rhinoceros (Rhinoceros unicornis) populations stabilized after reintroductions, with breeding successes noted since 2010. Prey species like hog deer and also recovered, supporting predator guilds, as evidenced by camera-trap data from long-term monitoring. Local Bodo communities, integrated into conservation via eco-development committees after 2003, aided this rebound by reporting poachers and promoting sustainable livelihoods, though ongoing vigilance is required against residual threats like cross-border incursions. Overall, Manas exemplifies post-conflict ecological restoration, with biodiversity indices approaching pre-insurgency levels by 2025.

Ongoing Threats and Policy Responses

Ongoing threats to the Manas River ecosystem include invasive weed proliferation, such as Mikania micrantha and , which degrade habitats and reduce native in the adjacent , as flagged by the IUCN's 2025 assessment categorizing the site under "significant concern." Illegal cultivation, grazing, and wildlife trafficking persist, exacerbating and endangering species like targeted for , with demand-driven remaining a key pressure despite post-conflict recovery efforts. Climate-induced alterations, including intensified flooding and erosion, pose the primary long-term risk, potentially amplified by upstream hydropower developments in , such as the Mangdechhu project, which have historically contributed to downstream sediment disruption and riverbank loss in as of 2003 events. Policy responses emphasize transboundary cooperation through the Transboundary Manas Conservation Area (TraMCA), established in 2011 across 6,500 km² linking India's and Bhutan's , prioritizing habitat connectivity and joint patrolling to counter and encroachment. The Manas Integrated River Basin Management Project (M-IRBM), funded by the , targets in the glacier-fed basin by improving services and addressing hazards through integrated strategies between and . Bhutan's Conservation Management Plan (2023–2033) integrates anti- measures, habitat restoration, and community involvement, while India's Manas Foundation supports revenue for anti-trafficking operations, contributing to predator population rebounds documented in 2025 surveys showing 57 adult s. Both nations have jointly canceled problematic upstream dam proposals, as affirmed in reports, to mitigate hydrological alterations, though enforcement against invasives and grazing requires ongoing bilateral monitoring.

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