Situated on the North Bank of the River Brahmaputra, the Dhemaji district
and Dhakuakhana Civil Sub-division is bounded by the Brahmaputra River in
the South and Subansiri River in the West. In addition, a number of
tributaries from the North join the Brahmaputra in this region one after
the other. This region covers one of the heaviest rainfall areas in Assam
due to which these areas experience regular annual floods, especially
after the Great Earthquake of 1950, which left the entire riverine system
of the area severely disturbed. During the months of May to September with
the onset of South East monsoon rains, huge volumes of floodwater starts
spilling all along the 720 Km. length of embankments of the River
Brahmaputra in Assam, out of which 132 Km. are within the jurisdiction of
Dhemaji E & D Division. The intensity of floods can well be imagined
during the months when the waters of the Brahmaputra synchronize with that
of the other tributaries.
Both naming and describing the course of rivers in
these areas are greatly difficult tasks. Many reasons can be assigned for
Frequent and rapid change of
Lack of knowledge on
regional geography among local people
Repetition and renaming of
Lack of hierarchical order
Lack of written historical
Difficulties in phonetical
Mingling of varied
Very complicated and
inhospitable riverine areas
Absence or lack of
toposheets of required detail
Names of some
major rivers and tributaries of the region
Other small tribulets
[Ref: Dhemaji E & D Report May – 2001]
The Dhemaji District exhibits many
paleochannels, ditches, swampy land, waterlogged area, rivers, natural
levees, waste land, etc. and their distribution itself speaks about the
devastation and extent of the flood problem. The shifting of channels is so
abrupt and uncertain that the whole district can be said to be the flood
plain area of some river. So, it is almost unscientific and risky to
demarcate as well as classify the land use pattern to be suitable either for
cultivation or for human settlement.
2. SOME IMPORTANT RIVERS OF THE REGION
The Brahmaputra originates at a place 30o31’N (31o30;
Assm. Dist. Gaz. Lakhimpur, 1976) latitude and 80o80’E (82 o
E; Assm. Dist. Gaz. Lakhimpur, 1976) longitude, near the upper waters of
the Indus and Sutlej, and a little to the east of the Mansarovar Lake in
Tibet between the Kailash range and the Himalayan range lying at an altitude
of 5150m. Then it travels along a trough north of the Himalayas running
parallel to the main Himalayan range before it comes out from a glacier
called Chema Yangdung. In the source region the river is known as
Matsang Tsangpo or Tamchok Khambab. Further downstream, it is
known as Tsangpo (means purifier). After flowing about 1700 km in
Tibet, the river enters into Arunachal Pradesh near Korba; sweeping towards
south around the Easternmost Himalayan Ranges at a few kilometers east of
Namcha-Barwa by the name of Siang in Arunachal through deep gorges,
rapids and cascades. The bending is well marked near the longitude 94o
E near Pemkoi Sung, ten miles below Gyala Singdon. In
this mountainous part, two left-bank tributaries - the Yange Chang Chu
and Yomme - and one right-bank tributary, the Siyom, joins the
Brahmaputra. It flows about 200 km in the region and appears in the plains
near Pasighat taking the name of Dihang. The Dihang then
follows a braided pattern for about 35-km before it meets the Dibang
from the North and the Lohit from the East near Kobo. From this
tri-junction, the river assumes a gigantic size and takes the name of the
Brahmaputra. The river then flows towards the Southeast and then due west
again in a braided pattern. It is so broad in these parts that its breadth
from one bank to another is sometimes 10 km wide. About 120 km downstream
from its tri-junction with the Dibang and the Lohit, just
below the confluence of Burhi-Dihing, the Brahmaputra bifurcates
giving out a fairly large branch in the Right Bank. This branch is known as
the Kherkutia upto the confluence of the Subansiri and then as
the Lohit. This branch receives several large tributaries in
Lakhimpur and reunites with the Brahmaputra at Luitmukh about 100 km
downstream. In between this branch and the Brahmaputra, there lies the
famous Majuli island. It is believed that a few hundred years ago, the
Brahmaputra flowed along the Kherkutia-Luit channel and the
Burhi-Dihing used to flow along the present course of the Brahmaputra.
But due to Southward shift, the Brahmaputra met with Burhi-Dihing and
the bulk of the former’s water began to flow along what was the lower course
of the then Burhi-Dihing.
Tributaries: The Brahmaputra receives many tributaries throughout its
length. The important right bank tributaries are the Jiadhal-Kumatiya
system, the Subansiri–Ranganadi–Dikrong–Buroi–Borgang –Ghiladhari–Jiabharali–Gabharu–Belsiri-Jiadhansiri
System, Pagladiya–Mora pagladiya–Tihu–Pohumara–Beki–Chaulkhowa–Moramanas–Manas-Ai
system, Champawati–Saralbhanga–Gaurang-Tarang system, and Gadadhar–Gangadhar-Sonkosh
system. Of the left bank system, the important ones are the
Dibru-Doomdooma-Dhola system, Sessa-Burhidihing system, Disang-Dimow
system, Dikhow-Janji-Teok-Bhogdoi-Kakodoonga system,
Dhansiri-Dipholu-Kollong-Kopili-Digaru system, Kulsi-Dudhnoi-Krishnai
system and Jingiram-Ghagra-Dilani system. The tributaries of the
Brahmaputra for most of their length drain the steep slope of the Himalayas
to the south where rainfall is heavy. Consequently they not only carry heavy
runoff, particularly where slopes are denuded of forest, but also very large
volume of detritus, the result of excessive soil erosion, to risen the land
surface in the plains by bank spill. The major portion of the heavily silt
laden floodwaters carried to the Brahmaputra not only aggravates its flood
congestion, but also adds to the silt charge of the river.
Drainage basin: The Brahmaputra drainage system covers an area of
1.75 Lakh km2 out of the total area of 2.25 Lakh km2
of the whole region. It covers the whole of Arunachal Pradesh, the northern
part of Meghalaya and the northwestern part of Nagaland. Most of its 1.75
Lakh km2 area receives an average annual rainfall of 200 cm. The
maximum discharge of the river at Dhubri is 0.7 million cusec.
The mass of silt brought down from the Himalayas is sufficient to form
sandbanks and even islands in the lower parts of the valley, wherever it is
blocked by any impediment in the river. Thus the characteristics of the main
channel alters and gives rise to an intricate network of waterways. Broad
streams diverse from the main river and rejoins it after a long separate
existence of uncontrollable meanders.
Cross-section: The dry season channel varies in width from 500 feet
to a few thousand feet. Large stretches of sand chars extend for
several miles at many places, until a bank sufficiently high is formed to
limit the width of the channel during the flood season, but not enough to
prevent inundation during floods. In the vicinity of Guwahati and Goalpara
the width of river is about 35000 ft. flowing between more or less permanent
banks with greatest depths varying from about 50-60 ft. during dry season to
130-150 ft. during rainy seasons.
Floods: Though the tributaries, sub-tributaries, streams & nalas have
great contribution to the overall flood situation in Dhemaji district, the
damage caused by the Brahmaputra is the greatest. The Brahmaputra behaves as
a braided channel near Dhemaji and further down stream. The alluvial
deposits within the main channel lead to sideward migration of the banks.
Therefore, besides the flood problem of the Brahmaputra, intensive bank
erosion is another problem. The primary cause of erosion has been
established to be the instability of the river, caused by the high volume of
silt charge, much beyond its carrying capacity. Between Kobo and Dhubri, the
south bank of the valley has undergone erosion over a length of 355 km. and
the north bank over a length of 230 km during the period 1923-1954.
Historical facts: During the regime of
King Kamleswar Singha (1709), a total placer gold deposit of 1,80,000 tolas
was extracted from placer deposits. The gold flakes collected from Subansiri
river sand was comparatively larger (maxm.
length 0.5" and breadth 0 .01”).
Alkali metal (%)
It is the first tributary in Dhemaji
district that debouches into the Dihang River. River Dihang travels only
about 2.5 km. through Assam before joining with Sillëy River along its right
bank. Remi river originating in West Siang District and Miku Korong that
travels from north in Arunachal Pradesh combine to take the name Sillëy and
travels downstream along south east. River Remi has a tributary named Penang.
Sillëy River travels 28 km. through Arunachal Pradesh and 10 km. through
Assam and combines with river Dihang in Assam.
River Leku in its upper reach in Arunachal Pradesh is known as Chilëng.
Chilëng River originates slightly west of the origin of Sillëy River. It
first travels eastward for some time, crosses the Pöva Reserve Forest and
then takes a southward bend. It takes the name Leku along the way and
combines with the Dihang near a ferryghat. The distance between Sillëy and
the confluence of river Leku and Dihang is almost 6.5km. downstream of river
Dihang. The length of river Leku is about 30 km. in Arunachal Pradesh and 10
km. in Assam.
To the south of the origin of river Chilëng,
river Rukshing originates and after flowing for some time in Arunachal
Pradesh, enters the plains of Assam near Jonai town and changes its name to
Jonai Korong. This river further travels towards south and meets a small
stream that bifurcates towards right from Dihang River at a distance 3km.
from the confluence of Leku and Dihang. The main Dihang travels further 8km.
from this confluence towards south and meets with the river Lohit and joins
with the Brahmaputra.
This is a
westerly-bifurcated branch of river Dihang. This branch separated out from
river Dihang at a distance 3km. downstream from the confluence of river
Jonai Korong and Dihang and after travelling 10 Km. along its western side
joins with the Brahmaputra. Streams like Yagalong, Rayang, Je or Rajakhana,
Debing or Berne Suti, etc. that originate at the foothills of Arunachal
Pradesh travel southward and debouch into the Buri Suti. After some time,
river Buri Suti again separates out from the Brahmaputra, travels about 60
km. towards west and again meets the Brahmaputra.
This river originates in
south West Siang district of Arunachal Pradesh as Sidha Korong River and
flows in the southwest direction for about 40 km. It receives the waters of
Përë Korog, Tepong , Dogë, Yungpi, Chipkai etc. rivers along the way and
after debouching into the plains area at Nari, assumes the name Dikari. It
travels about 12 km. along this plain course along southwest direction and
enters into the Jamjing Beel. In this Beel the Potë river,
which flows west of the river Dikari, merges with Dikari. Dikari River
travels through the Jamjing Beel for about 2 km. till it meets the
Dekapam River on the right side to give rise to Jamjing River. This river
further travels about 3 km. before bifurcating into two parts. The first
part flows southwards and merges with Buri Suti, while the other part
becomes Narod River, which flows further westward.
Simen River lies almost
in the middle of Dhemaji District. The river originates in the west Siang
district of Arunachal Pradesh, where it is joined by Nanyel river in the
left side and Jatë, Juri and Igo rivers along the right side, during its
almost 30 km journey downstream due southwest. The river Simen takes a
southward turn at a place 2 km north of Dipa Railway station where it
combines with Dipa or Sinyen River. Nikbum River then joins it along its
right side before it crosses the railway line. The river then bifurcates
into two streams - the main channel flows southward and the other part turns
towards east and enters a marshy land after being divided into three parts.
The main Simen channel combines with Nonarijan after travelling 1.5 km
further downstream (i.e., about 1km south of Dipa railway station), and with
Mirijan river after further 1km downstream along its right side. Bokajan is
a tributary of Miri Noi. About 2.5 km. downstream of this confluence, the
Simen River combines with Palë River, which travels along the southern
margin of Palë village of Arunachal Pradesh, near its debouching point. From
this confluence, Simen River flows further 5 km. downstream and again
bifurcates into two branches - One flows WS as Balikur and combines with
River Tongani and the other branch travels further southwards and merges
with Buri Suti, Southeast of Simenmukh.
Tongani/ Sissi river
At the upstream reach
several rivers combine together to form the Tongani River.
river combination: Jalakiasuti or Ghagra river flows along the west of
Baredi peak (701 m) of West Siang district of Arunachal Pradesh and flows
southwards 7 km downstream and enters Dhemaji District of Assam. Then it
travels along southeast direction for about 7 km before combining with
Silasuti Jan near Silasuti center along its left bank. Silasuti has a
tributary named Chalangjan. The Jalakiasuti River then travels in the same
downstream direction and combines with Dimow river flowing from north.
Dimow River is a
combination of some rivulets, e.g., Duin, Jarjee, Lutu, etc. near the north
east of Baredi peak. This river travels 12 Km. downstream along southeast
direction in Arunachal Pradesh and enters Assam near Sili of Dhemaji
District. During this course the river channel is filled up with sands and
gets bifurcated into two channels; one channel combines with Jalakiasuti
near Silasuti and flows further left and again combines with the combined
flows of Jalakiasuti and Dimow at a place further downstream. The Dimow
river and Jalakiasuti river combination is known as Archiyak river in their
downstream course .The river then flows through a marshy land during its
southward course for about 3km and combines with the left bifurcated branch
of Dimow along the left side. Balikur River, the daughter river of river
Simen, combines with Archiyak river after flowing another 7 km downstream
along its left bank, near south west corner of Muktiyar village. From this
point onwards the river is known as Tongani.
river combination: The Tongani river bifurcates near Deorighat after
flowing about 4 km downstream. One branch is known as Khirmi noi, which
travels 2 km south and merges with river Brahmaputra at a place east of
Deorighat. The other part remains as Tongani and flows due west, crosses the
road to Sonarighat and then turns south west after flowing 3 km west from
the road. The river thus flows 12 km downstream and is known as the Batua,
which combines with Sissi River. This Sissi River originates near
Sissiborgaon in the plain area of Assam. At its point of origin near
Kacharipathar it is known by the name of Gelua which crosses the NH 52 in a
southwest direction and then is known as Sissi River. It continues
downstream in the same direction for further 12 Km. after which it combines
with Batua (Tongani) near Batua mukh along its right side. This combined
river is then known as Sissi River. Near Nahorbari a distributary from this
river flows across its Right Bank and merges into Kapurdhowa River while the
main Sissi river travels downstream for another 7km. towards Borlung and
combines with River Brahmaputra at Sissi mukh.
There are two Sissi
rivers in Dhemaji district, the one that originates in the plain area of
Assam has been described in the previous paragraph. The second one
originates at Arunachal Pradesh and assumes different names, viz.,
Kapurdhowa, Laipulia, Charikaria or Charikora etc. in its lower reaches
before finally joining the Korha River.
According to 1970 data,
a river flows as Siji river in Arunachal Pradesh along SE direction and
combines with Siba, Sima and Yangtë river and takes the name Sissi river.
This Sissi river enters Assam about 1.5 km. north of Likabali town of
Arunachal Pradesh. This Sissi river flows 2.5 km. SW after this entry point
in Assam and joins with Balijan river along its right side then at further
1.5 km. downstream it combines with Nigaph river. After travelling further 1
Km. downstream, it is joined by Nilokh river near Nilokh Towani village. At
this point the river shows braided nature, wherefrom a distributary, known
as Nilokh Jan emerges along its right side, joins another river and takes
the name Biamjan.
The main branch of river
Sissi crosses the Railway Bridge after travelling 3 km. further downstream.
A kilometre further downstream, it divides its waters into two parallel
streams. One part flows as Gai River and the other as Sissi River, which
after flowing 5 km. downstream joins with Gutong river flowing from north
(near Kamchi) and enters the Chumani Beel. This Gutung River is the
combination of Biamjan and a branch coming from River Kanibil in the west.
Sissi River changes its
name to Kapurdhuwa after entering the Chumani Beel. Then it flows
through a marshy area towards SW and joins the Gai River near Kukura beel
along its right side. It combines with Kanibil River further downstream
and continues as Kapurdhuwa River along the eastern side of Pavomari beel
(4 km. east of Dhemaji) towards south and gets branches into two
channels. One joins with Batua in the east and the other takes the name
Laipulia and flows further 9 km. towards SW before it is joined by
Jorkatajan along its left side near Khajua to give rise to river Charikaria.
Laipulia River flows as Charikaria along SW direction and is joined by the
Harhi River along its right side and by Enduria along its left side. From
this point 3 Km. further downstream, Charikaria river combines with Marasuti
jan at Amulguri village. Still further 4km. downstream near Madarguri
village, it combines with Dhakuakhana jan along its right side at Bhagamukh.
This Dhakuakhana jan has a northeasterly flowing tributary, named Hapotia
jan, which also has a tributary named Singia jan.
Bhagamukh, Charikaria River flows SW for 1.5 km. before receiving Kharjan
along its left bank. It then crosses the Dhakuakhana town. Charikaria then
continues in the same direction for another 1 Km. and then combines with
Jogijan River that flows from Ghilaguri village along its left bank. Ekoria
or Juria jan is the tributary of Jogijan river. From this location
Charikaria flows further 12 Km. downstream and combines with Korha River
Thus the Sissi–Charikaria
River System includes Gai River, Moridhal River and Kanibil River in its
distributary emerges from Sissi River along southward direction at 2 Km.
north of Sissiborgaon near Tokoubari village. This branch, known as Gai
River, flows 2 Km. downstream and crosses the NH 52. From this point Gai
River continues through Salahani beel for another 4 Km. in the same
direction. Gai River flows leaving aside Chakamora and Chumani beel
on its left side. Near Chumani beel, River Gai changes its course
towards SW and travels 3 km. downstream, merges into Kukura beel and
combines with Kapurdhowa River after flowing across the beel. After
1970, a drastic change in the course of this river is recorded, which
requires a detailed scientific study to understand the root cause.
is the main river west of Sissi. Though Kanibil River looks larger and
broader but it is only a tributary to River Moridhal. Jiadhal or Kumatiya
originates almost at the same location in Arunachal Pradesh. These rivers
are, in fact, the triangular configuration of rivers in a swampy and
comparatively flat area at the foothills of Himalayas (alluvial fan). In
such geomorphological setting rivers do not follow the same course for a
long time and frequently change their course and the older courses become
either misfit, dry or marginalized channels. Moridhal is an example of such
phenomenon and the new course that it follows now as the main channel is
Jiadhal. Flash floods are common features of these rivers.
Moridhal River is formed by the combination of two rivers - Ronganoi,
originating in the Arunachal foothills, and Huliajan, originating in the
interior of the Arunachal Himalayas, near Laipuria village. The former flows
6 km. downstream towards east along the northern boundary of Huliajan
village northward of NH-52 at the Assam-Arunachal border, and combines with
Baruahjan, that flows from near Rampukhuri in the NW. From this point the
River Moridhal flows due SE for 1.5 km. downstream and joins with the
northwesterly flowing Borajan along its left side. 1.5 km. downstream of
this location, River Moridhal crosses the Railway Bridge and NH-52. It joins
with Kanibil River at a point further 2.5 km. downstream called Jamukguri.
Then it travels along SW direction for another 1.5 Km. distance where it is
joined along its Right Bank by Goriajan that flows from north of Lachan
village of Rai Chapori in the northwest. From this point Moridhal continues
along the eastern side of Goal Chapori for further 3 Km. and joins with
Kapurdhowa at Gohain Gaon 5 km. east of Dhemaji.
the Moridhol River
Kanibil River: The Rai River that originates at the foothills of
the Arunachal Himalayas and flows along SW direction receives Huliajan along
its left side. It also receives Ngopijan that flows from near Raiting
village close to the Assam Arunachal border near Borjan village along its
left side and takes the name Kanibil river. The river then flows in the same
direction for 3 Km. downstream and combines along its left bank with Kamia
River that comes straight from north near Perabhari village. This Kamia
River emerges from Arunachal hills and flows beside Gohaingaon towards
south. Kamia River has two tributaries - Kamtë and Kamphae Rivers. The
Kanibil River further flows 2 Km. downstream from Perabhari village and
combines with another small stream Kanibil jan along its left side. This
Kanibil is the combination of southward flowing Haew and Rangajan rivers.
Again two small branches get detached from this Kanibil River and merge into
Biamjan rivers in the south. This Biamjan then combines with Sissi River as
Gutung River in its downstream course. The combined Kanibil River then flows
along southeast direction for 1 Km. more and crosses the NH 52 and railway
line 20 Km. east of Moridhal Centre and then combines with Moridhal river at
Jamuguri village at a point 2.5 Km. further downstream.
the north of Dhemaji town, a paleochannel of river Moridhal is found. One
branch of this river merges into Pavomari River. Telijan River is another
branch that is found 1 Km. west of the above branch and 3 Km. north of
Dhemaji town. This Telijan River flows southward for 4 Km. and crosses the
NH 52 at Telijan Centre. This river receives Salikuchijan at Kaitang village
after flowing 5 Km. downstream along its left bank. This river is known as
Korha from Phatiha at further 1.5 Km. downstream. Korha flows southwards for
4 Km. downstream and receives Kaitangjan. Kaitang is also known as Laipulia,
Batua and Tongani at different places along its course. From the confluence
of Kaitang and Korha, the course becomes westward for 2.5 Km. upto
Benganagara Centre and turns towards southwest where it meets with Borjan
along its left bank. Balijan river, that emerges from Batua beel,
joins the Korha River on its right side at a point 3 Km. further downstream
near Madhupur tea estate. From this point, Korha River turns towards west
and joins with Naojan River just north of Dhenukhana village. After another
1.5 Km., the river becomes wider near Borbil, flows southwest and joins the
Kumotiya river near Batomari village after 4 Km. before crossing the
Ghilamara-Dhakuakhana road and then flowing 7 Km. through different beels
or swampy lands. The Korha River then meanders through the next 30 Km.
during which Bhathali jan joins it along its right side. After flowing
another 0.5 Km., it joins with Sampara Suti and changes its name to Sila
River. It flows as Sila River along south direction across Borchapori,
Dumachapori, etc. beels through a distance of 3.5 Km. and joins with
Charikaria River along its left side. Sila joins with Dangdhara River at a
point 4 Km. further downstream and travels through Bordol village for 5 Km.
before joining with Sungdiya beel and finally merges into Kherkutiya
Suti at a point opposite to Bordeuri village of Majuli.
river Jiadhal, a Northern Sub-tributary of the river Brahmaputra originates
in the sub-Himalayan mountains of Arunachal Pradesh at an altitude of 1247m
above the M.S.L. After passing through a narrow gorge in Arunachal Pradesh,
the river enters the plains of Assam in Dhemaji district where it flows in
braided channels. The river is known as ‘Kumotiya’ from the Railway line to
the Gogamukh – Ghilamara P.W.D. road wherefrom it is known as the river ‘Sampara’.
The river finally debouches into the river Brahmaputra near Selamukh. But
after construction of the embankment over the Kherkutiya Suti of the
Brahmaputra, the river falls into the Subansiri River.
total catchment area of the river Jiadhal is 1346 sq. km of which 306 sq. km
is in Arunachal Pradesh and 1040 sq. km is in Assam. The whole sub-basin
experiences heavy rainfall. The average annual rainfall as recorded at
Dhemaji is 3,500 mm.
The river Jiadhal carries heavy silt load
from the heavy catchment area during the flood season and deposits the silt
on its bed in the plains. Due to this fact, the riverbed has risen up
considerably. As a result the river follows a braided pattern and its width
is more than 3 km. in some of the reaches. The river is very much aggrading
in nature. This is why the river has a tendency to shift its course towards
the left bank.
In order to protect the flooded area on its
left bank, the embankment system on the left bank of the river were
constructed as below:
Flood prevention dyke of Jiadhal basin of length 5 km. constructed in
Extension of Jiadhal dyke from Basin to Tinigharia of length 5 km.
constructed in 1987-88.
M.E. along left bank of Jiadhal from Tinigharia to Rly. Line of
length 6.05 km. constructed in 1956-57.
In the year 1992, serious erosion
occurred on the left bank and the river avulsed causing breach of the dyke
at Sl. No 3 near Kekuri Village on 22.06.92. The river avulsed through this
breach and started flowing almost parallel to the railway line towards the
West. This breach has not been closed so far and the present length of the
breach is 1.335 km. (Ch. 690m to 2025m). Another breach occurred in this
embankment due to erosion near Dihiri Village on 05.07.98. This breach has
also not been closed so far and the present length of the breach is 800m
(Ch. 4000 – 4800m).
After the occurrence of the breach of the embankment
near Kekuri village, the railway line was working as an embankment for the
avulsed course of the river Jiadhal. Gradually the avulsed course started
affecting the Railway embankment. Finally, during the flood season of 1998,
the railway track near Samarajan was breached.
The change in geomorphic character of
Subansiri River has been recorded from 1919 onwards. Studies have revealed
four primary causes for the morphological changes in the river. These are:
Alteration of the direction of flow due to neck cut off.
Widening of channel in response to bar development.
Development (and subsequent abandonment) of anabranches.
Progressive shifting of meandering bends (without neck cut off) [Goswami,
Sarma, Patgiri, 1999]
The Subansiri is a Trans-Himalayan river
originating from the Western part of Mount Pororu (5059 m) in the Tibetan
Himalaya. After flowing for 190 km through Tibet, it enters India. It
continues its journey through the Himalaya of India for 200 km and enters
into the plains of Assam through a gorge near Gerukamukh. The Subansiri is
the largest tributary of the Brahmaputra. Its total length is 520 km and it
drains a basin of 37,000 km2. The river maintains an almost
stable course but becomes unstable as soon as it enters into the alluvial
plains of Assam.
In the10 km reach from the foothills near
Gerukamukh to Chauldhoaghat, the riverbed is composed of sand mixed with
pebbles and boulders. Further downstream, it is mostly composed of sand. The
average slope of the river bed from the foothills to Chauldhoaghat to the
confluence of Ranga river being about 24cm/km (0.00024). the river banks
from the foothills to Chauldhoaghat are composed mostly of sand, gravel and
silt, beyond which they are composed almost exclusively of alluvial silt.
The mean daily discharge of the Subansiri at Gerukamukh is 138842 m3/sec
(Goswami, 1997). The average annual sediment yield at Chauldhoaghat is 94.83
X 103 mtonnes (WAPCOS 1993). No significant anthropogenic effect
has been identified on the sediment or channel of the main river.
The major geologic event believed to be
primarily responsible for bringing about a change in the hydrologic regime
of the Subansiri River during contemporary times is the Great Assam
Earthquake and the associated historic flood of 1950. The earthquake caused
severe landslides in the mountains, and the Subansiri River channel was
temporarily blocked by rock debris at Sipomukh, about 2 km upstream from the
foothill regions. The huge naturally created dam obstructed the entire
monsoon discharge of the river for nearly three days. Subsequently, sudden
outburst of the dam created a catastrophic flood, the greatest ever in the
recorded history of the river. Sudden increase in water discharge and
sediment load altered the hitherto nearly stable hydrologic regimen of the
river bringing about other long-term changes in the river channel. The
gradual increase in the channel slope at the downstream reach of the river
appears to be another contributing factor responsible for a gradual
emergence of channel change.
Floods in River Subansiri
main river channel as well as its tributaries have shallow beds and do not
have well defined courses. Till 1976 records, the catchments area was forest
covered mountains with steep slopes. Rainfall is heavy. In the higher
mountains the overburden of snow is also deep.
Floods in Subansiri basin have three contributory factors viz. rainfall,
melting snow, and bursting of dams, which are formed by natural landslides.
The most important of all these factors is the heavy and prolonged rainfall.
The contribution made by snow melting is not very significant but due to the
prolonged nature of the monsoon in these parts, the periods of heavy
rainfall and melting of snow combine to cause heavier floods. The floods
released by the bursting of dams are more devastating on account of their
suddenness and extensiveness. In this case also, the menace of releasing
heavy floods in the Subansiri basin forming part of the wider Brahmaputra
basin may be considered to be present in a greater degree than in the basins
of other rivers whose mountain catchments are more stable and not so widely
encompassed by the zones of seismic activity. The gorges are narrow and such
susceptible to easy blockade. The floods, which came in the wake of the
great earthquake 1950, were caused by the blocking up of the river in the
higher gorge section at Sipolmukh and the sudden release of the stored
waters. During such floods the river gets charged with enormous quantities
of silt and in their movement downstream, alters the conditions of flow and
sometimes changes the river course. The dam across the Subansiri burst after
eight days of the earthquake and waves of flood water rolled down to its
basin engulfing villages causing death of 532 persons and innumerable
cattle. In Bordoloni and Kadam mauzas of Dhemaji and Lakhimpur districts
respectively, the flood havoc were the heaviest, followed by other mauzas,
Gohain, Nakari, Lakhimpur including the North Lakhimpur town, Telahi and
Kamalabaria mauzas. Other mauzas were also affected partially. The Ranganadi
bridge, which provided a vital link with some parts of the subdivision was
washed away. The devastation caused by floods of the Subansiri after the
great earthquake in 1950 was the most disastrous in the living memory of the
people of the North Lakhimpur and Dhemaji districts. There had been
extensive landslides after the great earthquake and courses of many rivers
were blocked. The natural dams so formed in the upper reaches of the
Subansiri, burst after a few days, as these could not withstand the pressure
of the swelling water. The huge mass of water rolled down engulfing the
villages and the entire region into a vast sheet of water. The level of
water in the river was 10 to 12 feet above the danger mark at some places.