Manas National Park and Barnadi Wildlife sanctuary are located at the foothills of the Bhutan Himalayas in Baksa, Chirang and Udalguri districts of Assam. The geology consists of tract zone with sandy loam soil with granite and igneous as bed rocks. There is a well marked Bhabhar tract of rocky soil towards the boundary of the National Park which contains sand stone, lime stone and shale, and have very little moisture retaining capacity. The Bhabhar tract generally remains drier. It is characterized by maximum relief, dissection and drainage density within it.
The formation is alluvial, comprising of mixture of boulders and sand, covered by debris, sand soil, silt and clay brought down by the rivers. Boulders are of granite, gneiss and quartzite and are well rounded to sub-round. The gravel deposits contain Granite, Gneiss, Phyllite, and Slate Schist. This leads to the formation of alluvial terraces, comprising deep layers of deposited rock and detritus overlain by sandy loam and then a thin layer of humus. Towards further south is the ill drained clayey Terai tract. These Teraitracts are more or less flat. The Terai tract in the south consists of fine alluvial deposits with underlying pans. The area of the Beki basin, in the west of the Park, is sometimes inundated during the monsoon but never for very long due to the sloping relief. The flood plain abuts against the dissected mountain front of the Sub-Himalayas.
Over the limestone and sandstone bedrock of the Bhabhar savannah area in the north, this has formed shifting river channels and swamps and a soil of porous alluvial terraces of coarse detritus under layers of sandy loam and humus where the water level is very low. The Terai grasslands in the south consist of deep deposits of fine alluvium with underlying pans where the water table lies very near the surface, making it potentially useful farmland. Elevation ranges from 50 m MSL on the southern boundary to 250 m MSL along the Bhutan hills.
The geological formations normally encountered towards the south of the Reserve are one of Sub-recent to recent origin and comprise of the following:-
- Bhabhar Formation:- This formation is represented by loose gravels, boulders and river deposits that are highly variable in composition and texture. This is also known as older alluvium. Sub-surface data indicate that clay is mixed in varying proportion and demarcation of distinct clay bed cannot be made. Except during monsoon, the formation is apparently devoid of water and the streams carrying water disappear in the porous structure of the formation and reappear further down south in the form of springs.
- Terai Formation:- This formation is transitional in character between the alluvial formation in south and Bhabhar formation in the north. It is better stratified and has more clay than in the Bhabhar formation.
- Alluvial Formation:- It is mainly represented by silt and clay without much gravel or boulder. It constitutes the flat plains to the South of the Reserve which is under extensive cultivation.
The soil is uniform throughout consisting of a light, friable loam varying in depth from 15 cm to 90 cm. In the uplands to the north of duars the soil is ferruginous clay.
The park is situated in the eastern duar and has extensive Bhabhar and some Terai areas, typical of Himalayan foothills. These Terai like tracts are more or less flat. The northern belt of the tract with major grassland areas of Manas is almost flat and well drained. The natural gradient of the land is a gentle sloping southward and area along the southern boundary is more flat and get water-logged during the rains. The river Manas, named Mangde Chu (in Bhutan) joined another stream Dangme Chu, flowing down through the Manas National Park and by splitting up into two major streams as Manas and Beki. The fast flowing Manas river on entering the plains from the steep hills loses its speed and as a result deposits enormous quantities of boulders, stones, sand, silt and other debris that are carried down by the water and this leads to the formation of alluvial terraces, comprising deep layers of rocks and sands deposits. The soil is deep alluvial Himalayan wash.
Hydrology and Water Sources:-
a) Natural:- The river Manas, named Dagme Chu (in Bhutan) joins another stream Mangde Chu, entering into Manas National Park splits up into three major streams known as Manas, Hakuwa and Beki, flows through the park and joins the river Brahmaputra some 50 kms further south. They carry enormous amounts of silt, rocks and debris from the foothills during monsoon. There are number of rivers and perennial streams that drain across the Park from north to south. In bhabbar tract most of the streams, nallahs and rivers dry up in winter. The Manas river, a major tributary of Brahmaputra, provides not only perennial water to the park but also forms crucial habitat of some fresh water fish and aquatic migratory birds and wild buffalos as well. Other perennial rivers like Burisuta, Pahumara, Rupahi, Semajhora, Satarupa, Angulichinga and some smaller streams also provide water in abundance even during dry months. Some small beels located inside the woodland areas of the bhabbar tract like ‘Daula Beel’, ‘Thekrai Beel’, ‘Garucharajhar Beel’, ‘Roisinglajhar Beel’ etc. serve as a perennial source of water even during dry period of the year. The hydrology of the area does not have any similarity with the flood plain protected area of the Brahmaputra valley.
The rivers, streams, ponds constitute water source of the National Park. The perennial rivers like Manas, Hakua, Beki, Burisuta, Angulisinga, Kasimdaha, Bangale-hat-dhua, Pota serve as the major source of water. In Bhabhar tracts most of the streams, nallah and rivers remain dry during winter. Other perennial source of water are: Pahumara, Rupahi, Semajhora, Satarupa, and host of smaller streams and natural ponds. Some bigger landlocked water bodies known as ‘beels’ are also available inside the park. Some important beels are: ‘Daula beel’, ‘Thekrai beel’, ‘Garucharajhar beel’, ‘Roisinglajhar beel’. During lean period rivers like Doimari, Garuchora, Rabang remain dry upstream at Bhabhar tract in India but provide water in abundance inside Bhutan territory.
One of the most important changes is the Manas river having changed its course in 2004 during the high flood season. It has resulted into major change in hydrology and compartmental change of habitat in Manas. The drastic change of river course of Manas that occurred in 2004 due to heavy flood is visible. Meandering through sub-Himalaya in Bhutan, the river Manas appears in the park as a single channel at Mathanguri and splits into many smaller channels forming distributive network system in the plains. This distributive network system forms a part of the crucial habitat of wild water buffalo, fresh water fish and migratory birds. The impact on wildlife was not ascertained. It emerges out of the park just west of Bansbari Range headquarter. Further, the impact on human populated areas on the down south outside the park was tremendous and devastating because of abrupt change of this river course. The situation assumed far more devastating during the high flood in 2007.
The river Manas originates in the Bhutan hills. It also carries enormous amount discharge from hills of adjoining Tibet. In hilly region a large number of sub-tributaries carry discharge to join on to a single river near the foot hill of Bhutan and to take the name Manas within Bhutan itself. The catchment of this river system covers nearly 50% of the geographical area of Kingdom of Bhutan over and above some area of Tibet also.
At the international boundary of India and Bhutan, the Bhutan-Manas debouches at Mathanguri into the plains of Assam with a sudden drop of gradient. This change in gradient result in energy dissipation and consequent silt, debris and boulder deposition for which the river spread and trifurcated. The trifurcated courses have their own names. The eastern most course is known as Beki, the central course is Hakua and the western most course retain the name Manas. Both Manas and Hakua has almost right angle inclination with respect to the alignment of Bhutan-Manas whereas Beki is almost aligned with Bhutan-India straight.
The river Beki is embanked at its left bank right from the foot hills at Mathanguri to old North Trunk Road for about 37 km of which the upper 15 km lies within the Manas National Park. As the name implies, this 15 km portion is covered with thick forest both at the C/Side and R/Side of the embankment.
List of rivers and nallahs inside Manas National Park:-
- Dhonbeel/Rupahi/Bangale hat-dhuwa/Pahumara
- Pota (Dry during the winter season)
- Hakuwa river
- Mora Manas
- Jiya pagla
- Lafasari nala
- Kanyasanti nala
- Artificial waterbodies.
There is no record of maintaining artificial water source in Manas National Park although there are signs on the ground that waterholes were created in the past inside the park. Occasional gully plugging of natural nallahs and streams are done during dry period to restrict the flow of water. A well-laid plan for improvement and development of existing water bodies to mitigate the shortage and scarcity of water in the event of drought is absent.