The Brahmaputra River, India
The Brahmaputra is one of the world’s largest rivers, with a drainage area of 580,000 sq.
km. (50.5% in China, 33.6% in India, 8.1% in Bangladesh and 7.8% in Bhutan). In India,
its basin is shared by Arunachal Pradesh (41.9%), Assam (36.3%), Meghalaya (6.1%),
Nagaland (5.6%), Sikkim (3.8%) and West Bengal (6.3%). Originating from the great
glacier mass of Chema-Yung-Dung in the Kailas range of southern Tibet at an elevation
of 5,300 m., it traverses 1,625 km. in China and 918 km. in India, before flowing 337 km.
through Bangladesh and emptying into the Bay of Bengal through a joint channel with
the Ganga. A unique river, it drains such diverse environments as the cold dry plateau of
Tibet, the rain-drenched Himalayan slopes, the landlocked alluvial plains of Assam and
the vast deltaic lowlands of Bangladesh.
In the plains of Assam and Bangladesh, the Brahmaputra flows in a highly braided
channel marked by the presence of numerous mid-channel and lateral bars and islands,
while in the Himalayan section its channel is steep and narrow with gradients as high as
14.8 m./km. An extremely dominant monsoon interacting with a unique physiographic
setting, fragile geological base and active seismo-tectonic instability together with
anthropogenic factors have moulded the Brahmaputra into one of the world’s most
intriguing gigantic fluvial system (Goswami, 1985; Ives and Messerli, 1989).
Geo-environmental framework
The Brahmaputra basin represents a unique physiographic setting vis-à-vis the eastern
Himalayas: a powerful monsoon rainfall regime under wet humid conditions, a fragile
geologic base and active seismicity. Flowing eastward for 1,625 km. over the Tibetan
plateau, the Brahmaputra, known there as the Tsangpo, enters a deep narrow gorge at Pe
(3,500 m.) and continues southward across the east-west trending ranges of the
Himalayas, viz. the Greater Himalayas, Middle Himalayas and sub-Himalayas before
debouching onto the Assam plains near Pasighat. These different geo-ecological zones
have a distinctive assemblage of topographical, geological, climatological and floral
characteristics. The gradient of the Brahmaputra river is as steep as 4.3 to 16.8 m./km. in
the gorge section upstream of Pasighat, but near Guwahati it is as flat as 0.1m./km. (see
figure on page 11). The dramatic reduction in the slope of the Brahmaputra as it cascades
through one of the world’s deepest gorges in the Himalayas before flowing in to the
Assam plains explains the sudden dissipation of the enormous energy locked in it and the
resultant unloading of large amounts of sediments in the valley downstream. Two rivers,
the Dibang and the Lohit, join the upper course of the Brahmaputra, known as the Dihang
(or Siang) river, a little south of Pasighat and the combined flow, hereafter called the
Brahmaputra, flows westward through Assam for about 640 km. until near Dhubri, where
it abruptly turns south and enters Bangladesh.
In the course of its 2,880 km. journey, the Brahmaputra receives as many as 22 major
tributaries in Tibet, 33 in India and three in Bangladesh. The northern and southern
tributaries differ considerably in their hydro-geomorphological characteristics owing to
different geological, physiographic and climatic conditions. The north bank tributaries
generally flow in shallow braided channels, have steep slopes, carry a heavy silt charge
and are flashy in character, whereas the south bank tributaries have a flatter gradient,
deep meandering channels with beds and banks composed of fine alluvial soils, marked
by a relatively low sediment load. Many of the north bank tributaries are of Himalayan
origin fed by glaciers in their upper reaches, e.g. the Subansiri, the Jia Bharali and the
Manas. The Debang and the Lohit are two large tributaries emerging from the extreme
eastern flank of the Himalayas, while the Jiadhal, the Ranganadi, the Puthimari, the
Pagladiya, etc. are major rivers having their sources in the sub-Himalayas, the latter two
in Bhutan. Among the south-bank tributaries, Burhidihing originates near the NagalandMyanmar border, the Dhansiri and the Dikhow in the Naga hills, the Kopili in the Karbi
plateau while the Kulsi and the Krishnai flow from the Meghalaya hills.
Due to the colliding Eurasian (Chinese) and Indian tectonic plates, the Brahmaputra
valley and its adjoining hill ranges are seismically very unstable. The earthquakes of
1897 and 1950, both of Richter magnitude 8.7, are among the most severe in recorded
history. These earthquakes caused extensive landslides and rock falls on hill slopes,
subsidence and fissuring in the valley and changes in the course and configuration of
several tributary rivers as well as the main course. In fact, the region’s active seismicity
has a significant impact on the hydro-geomorphic regime of the Brahmaputra system of
rivers, causing landslides that result in the natural damming of rivers, flash floods due to
the bursting of landslide-induced temporary dams, raising of riverbeds by siltation,
fissuring and sand venting, elevation of existing river and lake bottoms and margins,
creation of new water bodies and waterfalls due to faulting (Goswami and Das, 2002).
The earthquake of 1950, for example, raised the bed level of the Brahmaputra at
Dibrugarh by at least three metres leading to increased flood and erosion hazard potential
in the river (see figure on pg. 12).
Soils in the lower Himalayan ranges consist mainly of sands with an admixture of
cobbles and boulders and have a lower erosion threshold. The Brahmaputra valley is
made up of alluvial soils formed on recent river deposits called ‘new alluvium’. There are
a few isolated pockets of Pleistocene deposits called ‘old alluvium’ within the valley,
along the foothills and in parts of Bangladesh.
Climate regime
The Brahmaputra basin, excluding the Tibetan portion, forms an integral part of the
southeast Asian monsoon regime with a mean annual rainfall of 2,300 mm. Distribution
of rainfall over the basin varies from 1,200 mm. in parts of Nagaland to over 6,000 mm.
on the southern slopes of the Himalaya. The Himalayas exercise a dominating influence
on the prevailing weather of the basin due to their location in the path of the southwest
monsoon. Rainfall in the Himalayan sector averages 500 cm. per year with the lower
ranges receiving more. A gradual increase in rainfall from the valley bottom towards the
lower ranges followed by a decrease towards the higher ranges is evident from the annual
rainfall at Dibrugarh (2,850 mm.) in the far eastern part of Assam valley, Pasighat (5,070
mm.) in the foothills and Tuting (2,740 mm.) further up the Himalayas. Monsoon rains
from June to September account for 60-70% of the annual rainfall in the basin, while the
pre-monsoon season from March through May produces 20-25% of the annual rainfall.
Snowfall is experienced in the Brahmaputra basin in areas with elevations of 1,500 m.
and above. There are altogether 612 glaciers in the Brahmaputra basin of which 450 are
in the Teesta sub-basin of Sikkim while 162 are in the Kameng river (upper Jia Bharali)
sub-basin of Arunachal Pradesh.
Ecological base
The outstanding feature of the Brahmaputra basin is the great diversity of flora and
fauna and their enormous variation in both vertical and horizontal distributions. While
extraordinary luxuriance and great diversity in vegetation mark the monsoon-dominated
wetter parts, the ice-covered mountain-tops and other rain-deficient areas with thin soil
cover exhibit vegetation that is strikingly sparse or completely absent (Goswami, 1997).
Recent satellite surveys indicate that the Brahmaputra basin, as a whole, has a forest
cover of about 14.5%, grasslands occupy about 44%, agricultural lands about 14%,
cropland/natural vegetation mosaic 12.8%, barren/sparsely vegetated land 2.5%, water
bodies 1.8%, snow and ice 11%, urban land 0.02% and permanent wetlands 0.05%. The
total forest cover of the Brahmaputra basin in India is 1,14,894 sq. km. i.e. 54% of the
total area. The distribution of forest cover in the different states within the Brahmaputra
basin is as follows: Arunachal Pradesh (82.8%), Nagaland (68.9%), Meghalaya (63.5%),
Sikkim (38.1%), West Bengal (21.4 %) and Assam (20.6 %). As a whole, the eastern
Himalaya is more humid, its climate more conducive to tree growth with a relatively
higher tree line (average 4,570 m.) compared to the western and central Himalayas.
Recent satellite surveys indicate a considerable decline in forest cover due to
deforestation, land-use conversion and land degradation. Shifting (jhum) cultivation,
which is widely practised in the northeastern region and Bhutan is a major cause of
deforestation and has a disastrous impact on the region’s ecology through soil erosion,
loss of soil fertility, falling crop yields, lowering of groundwater recharge, increase in
surface run-off, sinking of the groundwater table and acceleration in the rates of
sedimentation in rivers and reservoirs downstream ultimately leading to a significant rise
in riverbed levels and intensification of flood hazards.
The Brahmaputra basin provides a unique habitat for an exquisite variety of fauna,
including many rare and endangered species. Among these, the great Indian one-horned
rhinoceros, pygmy hog, hispid hare, Asiatic elephant, clouded leopard, marble cat, golden
cat, binturong, hoolock gibbon, White-winged Wood Duck, Bengal Florican, etc. are
gravely endangered. The eastern Himalayan region is already identified as one of the
most threatened biodiversity hotspots of the world.
The northeastern region, especially the floodplains of the Brahmaputra, is dotted with
a large number of wetlands or beels, which possess tremendous ecological significance as
unique habitats for an exquisite variety of flora and fauna. The beels function as
floodwater retention basins and traditional fisheries. Over 3,500 such wetlands have been
identified in Assam, of which 177 are more than 100 ha. in size. Most of the wetlands are
now degraded while a considerable number have been totally destroyed due to siltation,
eutrophication, harmful land-use practices and encroachment for settlement, construction
of roads, railways, embankments, etc.
The Brahmaputra basin, with a total human population of about 75 million, is also
extremely rich in cultural diversity, with a great multitude of ethnic, socio-cultural and
linguistic groups occupying their unique habitats and ecological niches. The spatial
distribution and density of population is quite uneven in the basin, with the highest
density of 828 persons/sq. km. recorded in the Bangladesh portion, followed by India
(143 persons/sq. km.), Bhutan (26 persons/sq. km.) and Tibet (6 persons sq. km.).
Hydrological regime
The hydrological regime of the Brahmaputra is characterised by an extremely large
and variable flow, enormous rates of sediment discharge, rapid channel aggradations,
accelerated rates of basin denudation and unique patterns of river morphology.
The hydrological regime of the river responds to the seasonal rhythm of the monsoon
and freeze-thaw cycle of the Himalayan snow. With an average annual discharge of
19,830 m3s-1 (cubic metre per second or cumec) at its mouth, the Brahmaputra ranks
fourth among the large rivers of the world (see table on pg. 10). In terms of water yield
per unit discharge area, the Brahmaputra leads other major rivers, the rate for the
catchment upstream of Pandu being 0.0306 cumecs/sq. km. Water yields of some of the
tributaries of the Brahmaputra such as the Subansiri, Jia Bharali and Manas rivers are as
high as 0.076, 0.086 and 0.023 cumecs/sq. km. respectively (see table alongside),
surpassing many of the world’s major rivers (Goswami 1998). High monsoon rainfall in
the upper catchments and steep gradients are considered to be the major factors
responsible for the high rates of unit discharge, which in turn help generate the high
sediment yield from the basin and contribute significantly towards causing drainage
congestion in the valley.
The Brahmaputra is characterised by marked seasonality and high variability of daily
flow. The highest recorded daily discharge in the Brahmaputra at Pandu was 72,726
cumecs in August 1962 while the lowest was 1,757 cumecs in February 1968. The
discharge in the river between summer high flows and winter low flows fluctuates, on an
average, by 12 times although in certain years it has been as high as 20 times (Goswami
and Das, 2003). The mean annual flood of the river, 48,200 cumecs, has a recurrence
interval of 2.2 years, while the maximum recorded flood of 72,726 cumecs is likely to be
repeated once in about every 133 years. The enormously large variations in the river’s
daily discharge over different seasons is a remarkable feature of its flow regime. Since
the time lags and peaking characteristics of flood flows are different in different rivers
draining in to the Brahmaputra due to variations in catchment physiography and monsoon
precipitation, the tributary inflows generate large and variable perturbations on the
Brahmaputra’s discharge hydrograph.
Sediment characteristics
The Brahmaputra is one of the most heavily sediment-charged large rivers in the
world, second only to the Yellow (Hwang Ho) river in China in terms of the amount of
sediment transported per unit drainage area viz. 1,128 tonnes/sq. km./year at
Bahadurabad in Bangladesh. The river carries an average annual suspended load of 400
million metric tonnes at Pandu at an average daily rate of nearly two million metric
tonnes in the rainy season (May to October) accounting for more than 95% of the annual
suspended load (Goswami, 1985). Transport rates as high as 26 million metric tonnes are
recorded during peak flood flows. Estimates of sediment yield from tributary basins
indicate that 17 out of 32 rivers studied had rates in excess of 500 tonnes/sq. km./year,
the highest rate being 4,721 tonnes/sq. km./year from the Jia Bharali followed by the
Dibang basin with 3,765 tonnes/sq. km./year and the Puthimari basin with 2,887
onnes/sq. km./year (see table on page 11).
Sediment load and flow discharge in tributaries and the mainstream are observed to
demonstrate a high degree of correspondence although sediment transport in the river is
found to be more variable than its flow (Goswami, 1985). It is the moderate flows of
frequent occurrence that carry the bulk of the sediment load of the river as found from
sediment transport rates of the Brahmaputra river at Pandu (Goswami, 1988a). Flows that
recur at least once every two years carry more than 95% of the annual load while rare
events of larger magnitude recurring, for example, once in every ten years, carry a
meagre 3% of the annual load. During maximum annual discharges, the river is found to
carry only 1.3 to 3.3% of the annual sediment yield on an average.
Rates of bedload transport at Pandu, estimated by sediment discharge formulae, were
found to be highly variable and seemingly unreasonable (Goswami, 1988b) except for the
Laursen’s equation which shows that the rate of bedload carriage at Pandu is within about
5 to 15% of the total load in the river. This demonstrates an example of the kind of
uncertainties involved in quantifying sedimentation processes essential for designing
hydraulic structures including dams and reservoirs. The bank of the Brahmaputra, for the
most part, is extremely unstable. Bank failures are rampant and seem to be a function of
the hydraulic character of the river’s flow and the nature of the bank itself. Large-scale
slumping of banks observed during the falling stages of the river may be associated with
return flows in the permeable alluvium. High moisture content and low proportion of clay
make the banks highly susceptible to erosion.
The Brahmaputra exhibits drastic changes in channel configuration in erosion-prone
areas indicating the dynamic nature of river morphology and the intensity of erosion
caused by it. For example, at Palasbari near Guwahati, the bank line has shifted to the
south during 1911-1997 by different extents at different locations, the highest being
7.6 km. The low shear strength of bank materials and the vigorously dynamic flow of the
Brahmaputra have rendered its channel width highly variable. Probably the most notable
example of the river’s enormous erosion potential is Majuli island, the world’s largest
inhabited freshwater island, which has lost as much as 371 sq. km. of its landmass in the
last 50 years, reducing its area from 1,246 sq. km. in 1950 to 875 sq. km. in 1998. A
survey based on satellite imageries indicates that during the period from 1971 to 1995,
the bankline of the Brahmaputra has shifted more than a kilometre at places such as
Tunimukh, Sonowalgaon in the Majuli island.
The Brahmaputra is a classic example of a braided river with multiple channels
twining around numerous mid-channel and lateral sandbars, locally known as chars. As
the flow begins to rise with the onset of the monsoon, most of the islands are submerged
and the river then flows in a more or less straight channel. The most striking feature is the
continuous shift of the thalweg (deep channel) from one location to another within the
bankline, its movement being high in the rising stage (May to August), most erratic
during the falling stage (September to October) and minimal in the low flow stages
(Goswami, 1998). The mechanism of braiding may be attributed to factors such as
excessive sediment transport, large and variable flow, erosion-prone banks and the rapid
aggradation of the channel (Goswami, 1991; Leopold, 1964). The intensely braided
channel of the Brahmaputra in Assam is dominated by a profusion of bed forms ranging
from small-scale ripples to giant-sized dunes and sand waves.
Estimation of the amounts of aggradation and degradation in the Brahmaputra over
several decades reveal that as a whole, the Assam section of the river registered a net
bed-level aggradation of about 16 cm. per year although in certain sections, such as near
Dibrugarh, it was as high as three metres, subsequent to the 1950 earthquake.
Basin denudation
The catchments of the Brahmaputra and its tributaries show significantly high rates of
basin denudation especially after the great earthquake of 1950. The catchments of the
Subansiri, Jia Bharali and the Manas along with the Dihang (Siang) are estimated to
have experienced an average denudation of 73-157 cm./1000 years over just 24 years
(1955-79). The increasing amounts of sediment and water yields downstream indicate an
increase in sediment yield by a whopping 240% accompanied by an equally significant
rise of nearly 120% in water yield during the period 1971-1979 between Tsela D’Zong
(China) and Ranaghat (India). High rates of denudation of the Himalayan catchment of
the Brahmaputra are probably attributable to the rapid uplift of the mountain system,
steep slopes, high susceptibility to erosion, intensely powerful monsoon regime, recurring
earthquakes and the adverse impact of anthropogenic factors (Goswami, 1985). A
denudation rate of about 3 cm./1000 years from the eastern Himalayan basin of the
Brahmaputra during the last two million years has been estimated. This is lower by
several orders of magnitude compared to the present rate of 115 cm./1000 years.
Hazard scenario
The Brahmaputra basin, particularly in Assam, has earned notoriety for the awesome
hazards of annual floods and erosion, bringing misery to the people and shattering the
fragile agro-economic base of the region. With 40% of its land surface susceptible to
flood damage, Assam’s Brahmaputra valley represents one of the most acutely hazardprone regions in the country, having a total flood-prone area of 3.2 million ha. Floods in
Assam are caused by a combination of natural and anthropogenic factors. The unique
geo-environmental setting of the basin vis-à-vis the eastern Himalayas, the highly potent
monsoon regime, weak geological formation, active seismicity, accelerated erosion, rapid
channel aggradation, massive deforestation, intense land-use pressure and high
population growth, especially in the floodplain belt and ad hoc temporary flood control
measures are some of the dominant factors that cause and/or intensify floods in Assam.
The scenario is further exacerbated by a myriad of social, environmental and economic
factors that make populations increasingly vulnerable.
The state has experienced major floods in the years 1954, 1962, 1966, 1972, 1977,
1984, 1986, 1988, 1998 and 2002. In the aftermath of the great earthquake of 1950, the
intensity, frequency and the damage due to floods increased progressively. The floods of
1988 and 1998 were the worst in recent history. The 1988 flood broke all previous
records of flood damage, affecting 3.82 million ha., nearly 12 million people, five lakh
households and 1.43 million ha. of cropland, besides claiming 226 human lives and
innumerable cattle and wildlife. The total damage was estimated at Rs. 1,512 crores.
Similarly, the 1998 floods were unprecedented in terms of inundation and persistence.
Record high flood levels were created at many gauge stations . In the second wave, the
river flowed above danger level in the upstream reach between Dibrugarh and Neamati
for a record 100 days and in the downstream reach at Dhubri for 83 days.
All the 21 districts in the valley with 4.7 million people in 5,300 villages were affected,
damaging 0.97 million ha. of cropland. 30,400 houses were washed away or damaged
and 156 lives lost. The total loss was estimated at Rs.1,000 crores. In 2002, almost the
entire Brahmaputra valley was submerged; the loss was estimated at Rs. 2,000 crores.
Breaching of embankments has been a major cause of intensification of the flood
hazard in recent times. The undesirable consequence of embankments, especially in
aiding channel aggradation and overbank flooding is clearly visible in Assam. Structural
measures, mainly embankments, have been used so far as the sole answer to tackling
floods. Out of a total of 15,675 km. of embankments built in the entire country, Assam
alone has as much as 5,027 km., about 32% of the country’s total.
Water resources availability and utilisation
While it is true that the present utilisation of the colossal water resource potential of
the Brahmaputra river system is dismally poor, the policies and practices for the
utilisation of these resources in the future need to be have a broader outlook and a
changed paradigm and philosophy of development. As against the ad hoc, piecemeal,
short-term structural measures that are being adopted now, an integrated basin
management approach based on the principles of soil and water conservation as well as
sustainable development needs to be adopted.
We must go beyond the confines of techno-centric management to broader notions of
resource utilisation and disaster management. Proper assessment and reduction of
vulnerability, empowering local populations and strengthening existing institutions are
essential for this. For the Brahmaputra river with its diverse background of natural and
human heritage, such a plan may contribute immensely towards ensuring food, health and
ecological security.
As a long-term strategy for resource utilisation and hazard management, a judicious
mix of structural and non-structural measures with a greater emphasis on the latter should
form the core of the watershed-based regional plan. The water resources of the northeast
region must be planned zonally to ensure optimal resource utilisation, hazard
management and welfare maximisation (Goswami, 1992). Networking of hydrometeorological data centres across state, national and international boundaries can play a
key role in the Brahmaputra basin (Goswami, 2002). Being an international river of
immense size, huge resource base and high hazard potential, only effective cooperation
and coordination among the basin countries, together with persistent efforts at the
national and regional levels can create an effective response mechanism to the problem of
floods and usher in an era of progress and prosperity for the region.
Agriculture, the mainstay of most of the basin dwellers, suffers the most as a result of
gross under-utilisation of existing water resources and the impact of ravaging flood and
erosion hazards. In view of the extensive flood damage to paddy crops, there seems to be
an urgent need to adjust the crop calendar by increasing the emphasis on winter (rabi)
crops. However, the development of sustainable, localised irrigation facilities that
safeguard environmental and social concerns and the introduction of environmentally
benign and cost-effective methods such as water harvesting where participation of local
population can be ensured are prerequisites for any such change.
Conclusion
In view of the inadequate knowledge base, lack of systematic data over an adequate
timespan and across diverse terrains and considering the intense dynamism and immense
scale of the geophysical processes of the Himalayas, the wisdom behind constructing
large dams in the Himalayas raises more questions than can possibly be answered.
Besides, given the raging controversies over issues such as inappropriate assessment of
environmental and social impacts, lack of transparency and public participation in the
decision-making process and displacement of local communities and loss of their lands
and livelihoods, the question assumes further complexity.
The management of the Brahmaputra’s vast waters needs adequate regional, national
and international cooperation and pooling of resources and expertise. Strengthening of
the information base, assessment of available technologies vis-à-vis the unique nature of
the region and exploration for new appropriate strategies are other aspects that need to be
emphasised. What is most needed is a strong political will both at the state and national
levels and sustained popular zeal to convert the resources of the Brahmaputra into a force
for regional sustainable development through an integrated, multi-disciplinary approach
that covers not only technological but also social, economic and environmental aspects.
What bothers one the most today is the level of trade-off between a multitude of
seemingly valid concerns raised by critics and the apparently legitimate societal needs
that these structural measures are capable of satisfying in an underdeveloped region such
as the northeast. Under the existing circumstances of the region, modest interventions
with minimum impact on the environment appear to be the safest option. Any deviation
from this is fraught with uncertainties that may lead to grave consequences.
The northeast region drained by the Brahmaputra system of rivers possesses about
30% of the total water resources potential of the country, although the region represents
only 7.7% of the total Indian landmass. The per capita and per hectare availability of
water in this region is the highest in the country. The river system carries about 41% of
the country’s total hydro power potential, of which about 3% has so far been tapped for
human use. The region’s peak demand of power by the end of the ninth plan (2001-02)
was expected to be 1,809 MW, which would be met by a targeted generation of 2,419
MW. The planned addition to hydro power generation is 19,098 MW by 2020, which is
more than three times the projected peak demand of power in 2020 (5,700 MW).
Similarly, as against an ultimate irrigation potential of about 4.26 million ha., the area
presently under irrigation is only 0.85 million ha. and only 4.3% of existing groundwater
potential has been developed thus far. Availability of groundwater at relatively shallow
depth (within 20 m.) is also very high in the region, especially in the valley areas.
However, only 4.3% of the existing potential has been developed so far in the region.
River navigation in the 890 km. long reach of the Brahmaputra from Sadiya to the
Bangladesh border which is now the ‘National Waterway No. 2’ of the country has also
not been utilised to its potential. 