Topographically Assam is divided into two major divisions,i.e.,the plains and the hills. The plains are further divided into two units,the Brahmaputra Plain and the Barak Plain. The two plains are separated by the Karbi Plateaus and the North Cachar Hills. Similarly,the hills can also be divided into two units the Karbi Plateaus and the North Cachar hills. While the Karbi Plateaus are genetically Plateaus belonging to the Deccan Plateaus, the North Cachar hills are folded hills genetically akin to the Himalayas. Thus,it is seen that Assam can be topographically divided into (a)the Brahmaputra plain, (b)the Cachar Plain, (c)the Karbi Plateaus and (d)the North Cachar Hills. These four topographic units are briefly described below:


(a)The Brahmaputra Plain:


The Brahmaputra Plain(56,339 km2)is largely an alluvial plain with a length of about 772.45 km. and an average width of about 80.47 km. It is bounded on the north by the Bhutan and Arunachal Himalayas,on the east by the hills of the Patkai and its branches lying in Arunachal and on the south by the hills of Nagaland and the Plateaus of Karbi and Meghalaya. In its upper part,the plain runs from the north-east to the south-west, but in the lower part it rubs due east-west. The plain is at its widest in the upper part where it is about 90 km, and narrowest in the middle part where the Karbi Plateau projects north ward to the bank of the Brahmaputra at Burapahar, constricting the plain to a mere 50km.width.6Towards the west, the plain widens to became narrow again near Guwahati and opens up further west as the Garo hills of the Meghalaya Plateaus recedes southward. The Himalayan foothill margin in the north of the plain is fairly regular except where the plain embankments along the large tributaries like the Subansiri, Jia Bharali, Pagladia, etc.,enter into the mountain tract .The southern margin is, however, irregular with river valley entering deep

into the hills and Plateaus. For example,the plain extends eastward along the Buridihing valley upto as far east as Naw Dapha in Arunachal. Similarly, the Dhansiri, the Kapili and the Kulsi also have opened up valleys in the Plateaus widening the scope of the plain.


The plain as a whole slopes down towards south-west in the upper part and west in the middle and lower parts. But the gradient is extremely gentle. For example, the altitude of Kobo near the trijunction of the Dihang, Dibang and Luhit is 130 m. Then,96 km downstream at Dibrugarh the altitude is 103.6,m .Similarly, the altitude of Guwahati, Goalpara and Dhubri are only 50.5 m, 36.9 m and 27.8 m respectively. The fall of gradient is, as such, only 13 cm per km on the average. Such a low gradient is one of the principal causes of frequent occurrence of floods in the plains following heavy and continuous deluge in the season of south-west monsoon.


An interesting feature of the Plain is the presence of a series of hillocks on either bank of the Brahmaputra, especially in its middle and lower parts. These hillocks are made of old granitic and gneissic rocks and geologically part of the Karbi-Meghalaya Plateau. Looked from the east the hillocks begin to appear from Negheriting westward. Next hillock is seen at Biswanath in the north bank. Opposite to these hillocks,there are the ones in and around Tezpur town in the north bank like the Bhomaraguri, Agnipath, Bamuni Pahar, Da Parbatia, etc. Further west in the north bank there are Singari and Kurua hills and Mayang hills in the south bank. In Kamrup district, the Meghalaya Plateau skirts the Brahmaputra, especially near Guwahati and some of the hillocks and found scattered even in the bed of the river and in the north bank. The hills from Chandrapur to Nilachal in the south bank, Umananda, Karmanasha, etc, in the river bed and Mandakata-Changsari, Agaithuri, Garurachal, Barambai,and Hatimura in the north bank are such outcrops of the Meghalaya Plateau. Further west there are Dakhala and Nagarbera in Kamrup district in the south bank. Almost opposite to Nagarbera in the north bank there are Phulara, Chatala and Baghbor hillocks in Barpeta district,In Goalpara district,the important river bank hillocks are Hoolookauda and Pancharatna in an around Goal para town. Opposite to these hillocks in the north bank,there are hillocks in Bongaigaon district in an around Jogighopa township. It is interesting to note that some of such isolated hillocks are found to be scattered further north in the Bongaigaon and Kokrajhar districts reaching as far north as the railway line. Similar isolated hillocks are seen in Dhubri district a Rangamati and Mankachar. All the hillocks have their underground link with the Karbi-Meghalaya plateau and apparently seen to stand out and the flat alluvial plan.


Although the Brahmaputra plain is by and large flat,it has some measure of topographic variation. In the northern part along the Himalayan foothill, there is a narrow high zone made up of coarse river deposits. This zone is known

as the Bhabar Zone and supports dense forest. To the south of the Bhabar Zone and parallel to it,there lies the flat Tarai zone where the where the ground remains damp and sometimes springs ooze out. The Tarai zone is covered by tall grasses and was malarial in the past. To the south of the Tarai zone there lies a strip of relatively firm ground made of silty loam. It is this zone on which density of population is very high. This zone also holds most of the towns and commercial centres. Besides, the railway line and national highway pass over this strip that extends east-west. This trip may be refereed to as the built-up zone. To the south of this zone lies the following flood plain of the Brahmaputra supporting a series of beels,marshes and swamps. While the bed of the Brahmaputra itself is more than 3 km wide on the overage its floods-plain extends to a distance of 5 km to 10 km on each bank As the Brahmaputra is a braiding river,its water flows through several winding channels within its bed separated by patches of sandy shoals. Thus many riverine islands,some temporary and some semi permanent,locally known as chaparies and chars come into being.


The south bank plain is irregular in width, being very width in the upper part,moderately wide in the Nagaon and constricted westward. However,along the foothill region in the south-bank there are higher grounds known as terraces created by erosional activities of the rivers. These terraces are densely forested or occupied by the tea gardens in the upper part and by deciduous forest and dispersed settlements in the lower part. To the north of the belt of terraces,there lies a built-up zone(as in the north bank)where again settlements,roads,railways and towns and commercial centers are frequent. This zone to the north is replaced by the Brahmaputra flood-plain as described earlier. The isolated river-bank hillocks are mostly confined to the flood-plain zone.


(b)The Barak Plain:


Like the Brahmaputra Plain the Barak Plain is also of alluvial origin. It is the headward part of the larger Barak-Surma-Meghna Plain,the major part of which now falls in Bangladesh. It is surrounded in the north by the North Cachar hills, in the east by the Manipur hills and in the south by the Mizoram hills. It is open only to the west. The plain is about 85 km long from the east to the west and about 70 km wide wide and the average from the north to the south. Total area of the plain is about 6962 km. The gradient of the plain from the east to the west is very low(from 75m to 51m)and the river Barak flows over it sluggishly through an extremely meandering course.


The Barak plain is characterised by the presence of scattered isolated low hillocks locally known as rilas. Their height rarely exceeds 100 m. These rilas are geologically same as the hills of North Cachar or Mizoram. The middle part of the Barak plain through which the Barak flows is the lowest part and contains

numerous swamps or 'bills and jhils.


(c)The Karbi Plateau :


The hills and plateaus of Assam now covers two administrative districts,viz. North Cachar Hills and Karbi Anglong. The plateaus cover both Diphu-Bokajan unit the Hamren unit of Karbi Anglong, while the hills cover North Cachar Hills. The plateaus are geologically ancient and a part of the Deccan plateau,while the hills are young and geologically belong to he Himalayan group. Karbi Anglong itself is spread over two separated areas. Its Hamren sub-division is a part of the Meghalaya plain while the Diphu and Bokajan sub-division are located in the Karbi Plateau proper.

The Hamren sub-division is physiographically a part of the Jaintia hills of the Meghalaya plain and hence it is relatively low. Being drained by the head streams of the Borapani and Kapili rivers its northern part adjoining the Nagaon plain has wide flat tracts not exceeding 500m in altitude. But towards south there are ranges of hills which rise upto 800 m at places. Hamren Plateauis separated from the Karbi Plateau proper by the Kapili-Jamuna Plain.

The Karbi Plataeu proper is oval in shape and highly dissected along its margins. The central part of it is,however,high and has such peaks as Chenghehision (Singhasan :1359 m)and Dunbukso (1361 m).The plateau gives out many streams to the surrounding lowlands of Golaghat and Nagaon districts and there are terraces at places where these rivers emerge to the plains. These support tea gardens and reserve forests.


(d)The North Cachar Hills :


The North Cachar Hills although lie contiguous to the Karbi Plateau are geologically and structurally different from the latter. The hills here are made of folded sedimentary rocks and were raised during the later part of the Himalayan mountain building movement. The hills generally have NE-SW alignment and lie between the Karbi Plateau in the north and the Barak Plain in the South. It is in this district that the highest hill range of Assam,viz. the Barail range lies. It extends from the south-eastern boundary of Meghalaya Plateau and run across the North Cachar Hills district and Nagaland in the ENE direction and ultimately joins with the Patkai Range in the Indo-Myanmar border. The Barail Range within North Cachar Hills reaches an average height of 1500 m. Haflong town itself is atop a high hill of this range. it is not only the highest range in the State but it also acts as the divider of the Brahmaputra and the Barak river basins in the mid-eastern part of the State. The rocks of the region are less compact and hence the hill streams,fed by heavy rainfall have cut deep channels,giving rise to gorges and steep-sided hills in the region.