Assam - An Overview



Assam is bounded by Bhutan and Arunachal Pradesh on the north; Arunachal Pradesh, Nagaland and Manipur on the east; Mizoram and Meghalaya on the south and Bangladesh and West Bengal on the west. It is located at the central part of the north-east India, with an area of 78,438 sq. km of which, 77667.99 sq. km is rural and 770.01 sq. km is urban area. The state of Assam is situated in between 24°08′10″ N and 27°58′15″ N latitudes and 89°42′05″ E and 96°01′14″ longitudes. Topographically, Assam can be divided into two major divisions: Plains and Hills. Plains can further be divided into two physiographic divisions: Brahmaputra Valley & Barak Valley. The hills are also divided into two physiographic divisions: Karbi Plateau & Barail Hills.


Brahmaputra Valley: The Brahmaputra Valley is the largest unit in the state. It is largely an alluvial plain with a total area of 56,339 sq. km. It extends from east to west for about 720 km and average width of 80 km. It covers about 72% of the State’s total area. It is bounded on the north by the Bhutan and Arunachal Himalaya, on the east by Patkai hills and its branches lying in Arunachal and on the south by the hills of Nagaland and the plateaus of Karbi Anglong and Meghalaya. The Brahmaputra plain in its east-west direction has four distinct physiographic divisions viz. (a) The Northern foothills, (b) The North and South plains, (c) The floodplains and (d) The Southern foothills. The Brahmaputra, the biggest river in Assam and one of the biggest rivers in the world, rolls down the plain of Assam from east to west for a distance of 640 km up to the Bangladesh border.  This river is known as the Tsangpo in Tibet, the Siang or Dehang in Arunachal Pradesh and the Brahmaputra in Assam and Jamuna in Bangladesh. It traverses a total distance of 2906 km through the Tibetan Plateau, Himalayan Mountain and the hills and plains until reaching the Bay of Bengal in Bangladesh. The Brahmaputra river system drains a unique physical setting of diverse biotic environment with its major tributaries like Manas, Pagladia, Puthimari, Jia Bhoroli, Ranganadi, Subansiri, etc. on the north and Krishnai, Kolong, Kopili, Dhansiri, Dikhow, Burhidihing, Lohit, etc. on the south

Barak Valley: The Barak Valley is situated in the southern part of Assam. It is also of alluvial origin and surrounded by North Cachar hills in the north, in the east by Manipur hills, in the south by Mizoram hills and in west by Sylhet plain of Bangladesh. The valley is about 85 km long from the east to the west and about 70 km wide on average from the north to the south. It covers an area of about 6962 sq. km accounting for about 9% of the total geographical area of the State. It is characterised mainly by presence of numerous scattered low hillocks which is locally known as ‘Tilas’ with maximum height of about 100m. The Barak plain contains numerous swamps, ‘Beels’, ‘Jheels’, and rivers. The Barak is the second largest river in north-east India as well in Assam. It rises on the southern slope of the Barail Range near the border of Manipur and Nagaland. The river with a total length of 900 km from its origin to mouth covers an area of 52,000 sq. km while it traverses a distance of 532 km up to the Indo-Bangla border in India. In Assam, the Barak has a total length of 225 km and it drains the southern part of Assam which includes Cachar, Karimganj, Hailakandi and the southern part of Dima Hasao district. The important north bank tributaries of Barak river are Jiri, Siri, Madhura, Jatinga and Larang, while important south bank tributaries include Sonai, Rukni, Ghagra, Dhaleswari and Katakahal. Many ox-bow lakes are also present on both the banks of Barak River.


Karbi Plateau: It is the north-eastern extension of the Meghalaya Plateau. The whole region consists of two unequal hilly parts, which are separated by Kapili River and its tributaries like Diyung, Lumding, etc. The two hilly parts are: The Central Karbi Plateau, which is the eastern part of the plateau. It is about 7400 sq. km. The region is dome shaped, while the Hamren plateau, is the southwestern part of the plateau and all streams and rivers present here are tributaries of Kapili River. It covers about 3000 sq. km.

Barail Hills: The Barail Hills of Assam, which covers the entire Dima Hasao district, are actually the continuation of Barail Range towards west stretching from Tuensang across Nagaland. In the middle part of the Dima Hasao district, the Barail ranges attain their highest peak. It divides the N.C. Hills into two main parts namely Brahmaputra basin and Barak basin. The northern part falls under Brahmaputra basin and southern part falls under Barak basin. The main Barail range is like common water shed of both Brahmaputra basin and Barak basin. In southern region of the Karbi Plateau, the Barail range rises from 300m and attains a maximum height of about 1866m. Processes like weathering, erosion are very much active in this region due to which deep valleys and steep slopes are seen.


Soil is the most important resource and it serves as one of the prime requisites of life. Soils turn the land through their relative fertility, support all agricultural activities and the plant growth and thereby act as the most important element of the natural ecosystem. As regards the soils of Assam, geology (parent material), topography and climate seem to play a vital role in their formation. Therefore, under varying geological conditions, topographical characteristics and agro-climatic situation five major types of soils are found in the hills, piedmonts, plateaus and plains of Assam, viz. (i) Entisols, (ii) Mollisols, (iii) Alfisols, (iv) Ultisols and (v) Histosols.


Two types of Entisols are found in Assam- Udifluvents, i.e. Younger Alluvial Soils and Ustorthent, i.e.  Bhabar soils. The Younger Alluvial soils are found mostly in the flood prone areas and these soils are deposited by floods of the rivers carrying silt. The texture of the soil is sandy, silty or clayey-loam. These soils are often neutral to alkaline with a pH value of 5.5. They are rich in phosphate, potash, calcium and nitrogen and suitable for cultivation of rice, jute, pulses, mustard, potato, vegetables, etc.

Mollisols: Haplaquolls or Tarai Soil

Haplaquolls or Tarai Soils form in semi-arid to semi-humid areas. These soils are coarse alluvium formed by debris of pebbles, cobbles and coarse sand and have deep, high organic matter, nutrient-enriched surface soil and are acidic in nature. They are formed along the south of the Bhabar region with dense vegetation of eastern Himalayas and is known as Tarai zone. In Assam, these soils are found particularly in the northern parts of Kokrajhar, Bongaigaon, Barpeta, Nalbari, Darrang, Sonitpur and Lakhimpur districts.


Alfisols are acidic in nature with pH value ranging from 4.5 to 5.0. In Assam, three types of Alfisols are found.

Paleustalfs, Haplaquants (Older Alluvium): This type of soil is found mostly in the zone above the annual flood level and occupies the areas between the upper limit of Brahmaputra flood plain and fringe areas between Karbi Anglong and Kopili and Dhansiri valley. The old alluvium extends in certain parts of the flood plains of Kamrup, Nalbari, Bongaigaon, Kokrajhar districts and the tracts bordering hill ranges extending from north to south in Karimganj and Hailakandi.  The texture of the soil varies from coarse sand to clayey loam.

Haplustalfs, Paleustalfs, Rhodustalfs (Red Sandy Soil): This soil is formed sediments derived from crystalline metamorphic rocks which consist largely of granite and gneisses and occupies the area covering the Precambrian Plateau tracts of Karbi Anglong and the regions bordering Meghalaya with Kamrup and Goalpara districts. Red Sandy Soils are found in some isolated areas within the Brahmaputra alluvial plain in Kamrup, Kokrajhar, Dhubri, Bongaigaon districts, the southern part of the Borail Range in Dima Hasao district and the elongated tracts in southern Cachar district. The texture of the soil varies from coarse to fine sand and clay.

Haplustalfs, Paleustalfs, Rhodustalfs (Red Loamy Soil): The Red Loamy Soil is comprised of sand, silt and clay mixed pebbles brought down by rain from higher slopes. It is found as elongated tracts lying in the north-south direction in Cachar, Hailakandi and Karimganj districts. It is also found towards the Assam-Meghalaya border areas in Karbi Anglong and Dima Hasao districts.


Ultisols are highly acidic with pH value varying between 4 to 4.5 and poor in plant nutrients. This type of soil is classified into two groups:

Palehumults (Brown, Red and yellow soils): These soils are found in long strips of land between border areas of Bhutan hills and districts of Darrang, Kamrup, Nalbari, Barpeta and Kokrajhar. Another patch is seen bordering Arunachal Pradesh in either sides of Subansiri river. This type of soil is derived from oxidation and hydrolysis of Iron minerals and comprises sandy clay originated from sedimentary formation.

Plinthaqualts, Plinthustulcs, Plinthudults (Lateritic soil): The Lateritic soil is highly leached soil, developed under the conditions of high rainfall with alternating dry and wet period. It occupies the area situated between old alluvium and red loamy soil in south and south-western part of Golaghat, south-eastern part of Nagaon district and almost northern part of Dima Hasao district. The Lateritic soil is also found between the Barak alluvial plain and foot hills of Barail Range bordering Cachar with Dima Hasao district.


Troposaprists (Peaty and saline Peaty Soils): This type of soil comprises silt and clay materials. It occupies the low-lying areas of the Brahmaputra flood plains and the riverine tracts of Lakhimpur, Barpeta and Jorhat district of Assam. It is also found in Barak flood plains especially in the western part of Barak alluvial plain in Karimganj district.   


Climate of Assam is governed by a number of factors which include orography, alternating pressure cells of north-west India and Bay of Bengal, periodic Western disturbances, local mountain valley winds, geological configuration, extensive water bodies, local depression and forests. The climate of the state is characterised by high humid tropical climate with heavy rainfall and moderate temperature. The greater part of two hilly districts, viz. Karbi Anglong and Dima Hasao, enjoys sub-tropical monsoon climate. The climatic year in Assam is divided into four seasons: Winter, Pre-monsoon or Summer, Monsoon or Rainy and Post Monsoon or Autumn.


It extends from December to February. This season is characterised by the northerly and north easterly cold winds and damp fogs. Besides, foggy morning, dry days along with misty and cold long nights are other general characteristics of this season. Average minimum temperature during this period is around 7ºC and the average maximum temperature is around 24ºC in the Brahmaputra valley, while slight variation is being noticed in the Barak Valley and the hilly districts of Assam. The minimum temperature generally remains around 11ºC at an average while average maximum  temperature remains around 26ºC in the Barak Valley, whereas in Karbi Anglong and Dima Hasao, it goes down to 5.5ºC and rises to 23ºC. Rainfall is scarce and its does not exceed 3 cm at an average. The Relative Humidity varies from 60% to 90% in average. During the early part of this season the deciduous trees shed their leaves and start leafing in the latter part of February. 


Pre-monsoon or Summer

It extends from March to May. During this season the temperature rises, fogs gradually disappear and at the later part of the season there is occasional thunderstorm and often the weather remains windy and dusty. The wind usually blows south-west to north-west and is often accompanied by cyclonic storm and torrential rains. Average rainfall during the months of March, April and May is found to be 60, 190, 240 cm (mean) respectively. The maximum temperature varies from 29ºC to 32ºC and minimum temperature varies from 24ºC to 26ºC.

Monsoon or Rainy

It extends from June to September. The season is characterised by cloudy weather, more or less incessant rain with heavy precipitation and high atmospheric humidity. Though there are more rainy days in the season, the temperature rises to maximum of 38ºC. The State of Assam receives an average annual rainfall of 200 cm. It is found to be as high as of 335 cm and 323 cm respectively in North Lakhimpur and Silchar and as low as of 129 cm in Lumding.

Post Monsoon or Autumn

It extends from October and continues till the onset of winter. Clouds start disappearing with progressively clear weather and gradual fall of temperature. The weather is quite stable and is often accompanied by mist and fogs with the advancement of the season. The temperature does not rise beyond 28ºC, rainfall is negligible and humidity ranges from 78% to 90%.


Vegetation of Assam is primarily of tropical and sub-tropical types. On the basis of floristic composition, the forest area can be divided as follows:

Tropical Forests

(a) Tropical wet evergreen forests: This type of forest lies along the foothills of Arunachal Pradesh, Lakhimpur, Tinsukia, Dibrugarh and southern parts of Sivasagar district adjoining Nagaland and the Barak Valley, foothills of Barail ranges and south-eastern parts of Cachar district comprising Loharbund Reserved Forest. Often, single species dominance in the top canopy are observed such as Dipterocarpus retusus, Mesua assamica, Mesua ferrea, Shorea assamica, etc. in upper Assam while Dipterocarpus turbinatus, Mesua floribunda, etc. in Barak Valley. Characteristic floristic elements include: Altingia excelsa, Dipterocarpus retusus, Dipterocarpus turbinatus, Magnolia sp., Mesua assamica, Mesua ferrea, Schima wallichii, Shorea assamica, Stereospermum tetragonum, Terminalia chebula, etc. These tree species form the main canopy. Smaller trees and shrub species found in this type of forest include: Bauhinia sp., Cinnamomum sp., Clerodendrum sp., Holarrhena pubescens, Ixora sp., Pavetta indica, etc. Lianes and climbers which include: Acacia sp., Entada rheedii, Gnetum montanum, Pothos sp., Thunbergia laurifolia, etc. while Musa sp., Phrynium sp., Arenga sp., Calamus sp. and Pinanga sp. are common in damp slopes.

(b) Tropical semi-evergreen forests: These forests are found in the Brahmaputra Valley and along the foot hills of Barail ranges and Manipur hills in the Barak Valley. In these regions, the forests are characterised by emergent deciduous species. Common tree species include Phoebe goalparensis in the western Assam districts and Phoebe cooperiana and Phoebe attenuata in Sivasagar and other similar localities. Other tree species include Castanopsis indica, Dillenia indica, Dillenia scabrella, Dysoxylum sp., Michelia champaca, Schima wallichii, Cedrela toona, etc. along with deciduous species like Albizia lebbeck, Albizia procera, Bauhinia purpurea, Dillenia pentagyna, Shorea robusta, etc. Clerodendrum glandulosum, Gnetum gnemone, Litsea sp., Morinda angustifolia, etc. are some of the shrubby species.  Apostasia odorata and Apostasia nuda are the two interesting ground orchids which remain restricted to the vegetation of Garampani and Joypur Reserved Forests. Epiphytic flora is also rich in these forests. Numerous species of Orchidaceae, Asclepiadaceae, Piperaceae are some common families found here.

(c) Tropical moist deciduous forests: These forests comprise the Sal forests and major parts of scrub forest in the state, which are seen in the districts of Kokrajhar, Goalpara, Kamrup, Nogaon, Dima Hasao and plains of Barak Valley. The dominant species in the Sal forest areas is Shorea robusta. Other species include Derris robusta, Dillenia pentagyna, Gmelina arborea, Lagerstroemia parviflora, Schima wallichii, Cedrela toona, etc. Woody climbers and lianes are scarce. The common ones include Combretum sp., Bauhinia vahlii, etc.

(d) Grasslands: These are grass dominated biomes. Extensive areas of Assam are covered by grasslands in alluvial flood plains of Brahmaputra Valley. The grasslands are most conspicuous in all the National Parks of Assam and most of the Wildlife Sanctuaries of Assam viz., Sonai Rupai, Laokhowa, Pabitora, Bornadi, Burachapori, etc. Besides these grasslands also occur in the riparian vegetation, marshes and swamps, marshy depressions, beels and hoars. Vegetation studies reveal that there are two different types of grasslands in Assam: (1)  Grasslands occuring in recent alluvial deposits in low lying and Char areas annually inundated and flood water retained for a considerable period of time. These are wet alluvial pure Grasslands. Some of the dominant grasses are Apluda mutica, Arundo donax, Brachiaria villosa, etc. (2) Grasslands in old alluvium deposits of high land. The dominant grasses include : Apluda mutica, Digitaria abludens, Imperata cylindrica, Neyraudia reynaudiana, Panicum auritum, Saccharum arundinaceum, etc.

(e) Riparian Forests: These forests occur all over the evergreen and semi-evergreen zones in Assam along river banks and extended in several areas. In low lying areas, species like Albizia lebbeck, Dillenia indica, Dalbergia stipulacea, Semecarpus anacardium are some of the common species while on slightly higher level, species like Mesua ferrea, Michelia doltsopa, etc. are found. A variety of orchids and ferns are the major component of the epiphytic flora in these areas.

(f) Swamps: The total wetlands have been estimated to be 2.1% and occupy an area of 1,036.76 sq. km. i.e. about 1.32% of the total geographical area (78,523 sq. km.) of the State. The plants growing in wetlands are usually termed as Hydrophytes. Aquatic families such as Araceae, Cyperaceae, Eriocaulaceae, Lemnaceae, Nymphaeaceae are common in these beels. Common species of the genera in this type of vegetation include Cyperus sp., Nelumbo sp., Nymphaea sp., Ottelia sp., Scirpus sp., etc.

Sub-tropical Forests

(a) Subtropical Broad-leaved Forests: These forests are confined only to places where elevation rises above 900 m and mainly found in Hamren Sub-Division of Karbi Anglong district and Dima Hasao district. Ascending upwards from 900 m – 1700 m the vegetation is predominated by broad-leaved plants. Common species include Alseodaphne petiolaris, Antidesma bunius, Chaetocarpus castanocarpus, Cinnamomum tamala, Ficus albelii, Ficus squamosa, Glochidion zeylanicum, Glochidion sphaerogynum, Helicia robusta, Litsea nitida, Phoebe lanceolata and Schima wallichii. Associated species of shrubs and undershrubs are less in number, some of which are Alchornea tiliifolia, Boehmeria clidemioides, Eriosolena involucrata, Oreocnide frutescens, Pouzolzia sanguinea, etc. The herbaceous angiosperms chiefly belong to members of Asteraceae, Lamiaceae, Araceae, Zingiberaceae and Poaceae. Higher up beyond 1700 m the broad-leaved vegetation is replaced by pure stand of pine forests but not dense. The species here is Pinus kesiya. Associated species of other angiosperms are less in number and mostly with species like Schima wallichii, Betula alnoides, Quercus semiserrata. Annual burning of pine needles affects the undergrowth vegetation much. Only a few hardy herbaceous plants of the families viz, Asteraceae, Rosaceae and Theaceae are found scattered.

(b) Subtropical Pine Forests: These forests occur at an elevation from 900 -1800 m, having moderate rainfall and found in areas of Dima Hasao bordering Jaintia hills district of Meghalaya and Karbi Anglong district. The pine forests i.e. mainly Pinus kesiya are intercepted by tree species like Cinnamomum sp., Engelhardtia spicata, Myrica esculenta, Quercus griffithii, Schima wallichii, Symplocos sp., etc. The floor is generally devoid of plant growth due to deposition of pine needles and very few plant species occur.

(c) Bamboo forests and cane brakes: There is as such no bamboo forest in both the valleys of Assam. Bamboo brakes are mostly found along the edges of Reserved Forests and residential compounds, often cultivated by the villagers, with scattered distribution of a few species. Pure bamboo forests are found in the two hill districts of Assam and mostly along the foothills of Barail range near Jatinga of Dima Hasao and in hilly slopes of Karbi Anglong district predominated with Chimonobambusa callosa and Melocanna baccifera. Langting Mupa Reserved Forest of Dima Hasao and in parts of Karbi Anglong are mostly with Melocanna baccifera in association with Bambusa tulda, Dendrocalamus hamiltonii, Dendrocalamus hookeri, Dendrocalamus strictus, Gigantochloa parvifolia, Phyllostachys mannii interrupted by moist semi – evergreen and deciduous species. Similarly in slightly more wet places different species of Calamus such as Calamus erectus, Calamus floribundus, Calamus tenuis form thick patches in Karbi and Dima Hasao hills.

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