Rivers of Dhemaji and Dhakuakhana (original) (raw)

1.INTRODUCTION

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 this inconvenience:

� Frequent and rapid change of river courses

� Comparatively recent settlement areas

� Lack of knowledge on regional geography among local people

� Repetition and renaming of rivers

� Lack of hierarchical order of naming

� Lack of written historical reviews

� Difficulties in phonetical assimilation

� Mingling of varied linguistic groups

� Very complicated and inhospitable riverine areas

� Absence or lack of toposheets of required detail

Names of some major rivers and tributaries of the region

1. Brahmaputra

2. Silley

3. Sibia

4. Leko

5. Jonai Korong

6. Dikhari

7. Narod

8. Somkhong

9. Tongani

10. Burisuti

11. Simen

12. Dimow

13. Gainadi

14. Moridhal

15. Jiadhal/Kumotia

16. Korha/Sila

17. Charikaria

18. Nonoi

19. Sampara Suti

20. Subansiri

21. 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

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 94oE 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.

Channel bars: 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�).

Silley river

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 Sill�y 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 Sill�y and travels downstream along south east. River Remi has a tributary named Penang. Sill�y River travels 28 km. through Arunachal Pradesh and 10 km. through Assam and combines with river Dihang in Assam.

Leku river

River Leku in its upper reach in Arunachal Pradesh is known as Chil�ng. Chil�ng River originates slightly west of the origin of Sill�y River. It first travels eastward for some time, crosses the P�va 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 Sill�y 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.

Jonai Korong

To the south of the origin of river Chil�ng, 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.

Buri Suti

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.

Dikari

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 P�r� Korog, Tepong , Dog�, 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 Pot� 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

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 Jat�, 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 Pal� River, which travels along the southern margin of Pal� 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.

Ghagra�Silasuti�Dimow 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.

Tongani�Batua-Sissi 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.

Sissi river

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 Yangt� 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.

Charikaria River

The 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.

From 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 near Machkhowa.

Thus the Sissi�Charikaria River System includes Gai River, Moridhal River and Kanibil River in its upper reach.

Gai River

A 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.

Moridhal River

This 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.

Tributaries of 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 - Kamt� 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.

Korha river

In 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. b_eels 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.

Jiadhal River

The 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.

The 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:

1. Flood prevention dyke of Jiadhal basin of length 5 km. constructed in 1976-77.

2. Extension of Jiadhal dyke from Basin to Tinigharia of length 5 km. constructed in 1987-88.

3. 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 Subansiri

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:

1. Alteration of the direction of flow due to neck cut off.

2. Widening of channel in response to bar development.

3. Development (and subsequent abandonment) of anabranches.

4. 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

The 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.