Short Notes on the Ganga Plains of Yamuna River: by - K.Raja
Short Notes on the Ganga Plains of Yamuna River
The Ganga Plains extend from the Yamuna river in the west to the western borders of Bangladesh covering a distance of about 1,400 km and an average width of 300 km (between the Siwaliks in the north and the Peninsular Uplands in the south).
It occupies a total area of about 3, 57,000 sq. km. and is drained by the Ganga and its tributaries like the Yamuna, and Son in the south and Ramganga, Ghaghara, Gandak and Kosi in the north. 'All through its course in the plains, the river is a braided stream bordered by low-lying depressions which get flooded during rains.' (Sharma and Coutinho, 1988, p. 6).
The plain has a general gradient of about 15 cm per km from north-west to south-east although there are many micro-slopes. The maximum height is found near Saharanpur (276 m) from where it goes on decreasing towards the Sagar Islands, i.e. Roorkee 274 m, Delhi 216 m, Meerut 222m, Aligarh 187 m, Agra 169 m, Hardoi 142 m, Kanpur 126m, Lucknow 111 m, Allahabad 98 m, Varanasi 76 m, Patna 53 m, Bhagalpur 49 m, Barddhaman 32 m, Kolkata 6 m, and Sagar Island 3 m.
The main physiographic variations in the plains include bhabar, terai, bhangar, khadar, river bluffs, dead channels, deferred junctions or bhils and badlands etc. Following L.D. Stamp, O.H,K. Spate and R.L. Singh the Ganga Plains may be divided into three smaller physiographic units :
(a) The Upper Ganga Plain-This region is bordered by the Yamuna river in the west and the
south, the Siwalik hills in the north and 100 m contour line (Allahabad-Faizabad railway line) in the east (73° 3'E-82° 21'E and 25° 15'N-30°-17'N). It occupies a total area of 1, 49,029 sq. km. which is about 63% of the total area of Uttar Pradesh.
The region has an average elevation between 100 and 300 m with average slope of 24 cm per km towards south-east. Besides the Ganga and Yamuna other important rivers of the region include Ramganga, Gomati, Ghaghara and Rapti etc. As the gradient decreases, drainage becomes more sluggish towards south-east with numerous abandoned courses, oxbow lakes and sandy stretches.
Although featureless plain the topographic prominences include a 32 km wide zone of unassorted sediments along the foot hills of the Siwaliks called Bhabar, followed by a zone of seepage with fine sand, silt and clay (called Terai), the ravine landscape or a bad land topography in the Yamuna-lowerChambal tract, the areas of older alluvium beyond the flood level (the Bangar or Uparhar), the areas of lower alluvium susceptible to frequent flooding (the Khadar or Tarhar), the undulating sandy uplands (called Bhur) along the eastern bank of the Ganga in Moradabad and Bijnor districts, the river bluffs (levees), and the abandoned channels forming ox-bow lakes and meandering courses of the rivers.
The micro-level topographic facets and their regional characteristics render it possible to divide upper Ganga Plains into three micro units: (i) the Ganga-Yamuna Doab, (ii) the Rohilkhand Plain, and (iii) the Avadh Plain. In the Ganga-Yamuna Doab the surface alluvium, which rests on the Siwaliks and the early Tertiary sediments, has a thickness of 1000 to 2000 m.
The general slope of the land is from north to south, but there are many micro slopes. The old Bhangar alluvium of the Doab has formed flat uplands distinguishable from the Khadar lowlands of the neweralluvium. The intervening slopes, which are often quite pronounced with relative variations of 15 to 30 metres in relief are, locally known as Khols. Two distinct alluvial terraces have been identified-a younger and an older one-both in the Ganga and the Yamuna Khadar.
They are flanked by two natural levees. The older levee has been more extensively eroded. Another unusual topographic feature of the Upper Doab is the aeolian Bhur deposits. East of the Ganga-Yamuna Doab are the low-lying.
Rohilkhand Plains drained by the Ramganga. Here Tarai in the North is quite extensive and presents well-marked variations in physical as well as cultural features. Furthereastward lies the Avadh Plains which is drained by the Ghaghara and the Gomati rivers. The former is famous for shifting its course and causes devastating floods. Here rice is the predominant crop.
(b) The Middle Ganga Plain-The Middle Ganga Plain includes eastern Uttar Pradesh (east of Allahabad-Faizabad railway line) and the Bihar plains (24°30'N-27°50'N and 8I°47'E - 87° 50'E) covering a total area of 1, 44,409 sq. km.
The northern boundary of the region runs along the Indo-Ncpal international boundary, while the 150 m contour marks the southern boundary along the Vindhyan - Rohtasgarh-Chotanagpur plateaux. The western boundary runs along 100 m contour while the eastern limit corresponds to the Bihar-Bengal state boundary.
The maximum length from cast to west is about 600 km while its width from north to south approximates 330 km. The surface is extremely low and level with heights varying from 100 m in the northwest to 75 m in the north-east and 30 m in the southeast.
The general slope north of the Ganga (20 cm/ km) is from north-west to south-east while in the south of the Ganga, it is from south-west to northeast (9 cm/km).
The average thickness of the alluvium lies between 1300 and 1400 m but the depth increases up to 8,000-10,000 m along the Himalayas. In fact, there are two large troughs, which may be callcd Gorakhpur trough and Raxaul-Motihari trough of over 8,000 m deep.
South of the Ghaghara in the Eastern U.P. Plain and the Bihar Plain the depth of alluvium falls below 1500-3000 m, except a few patches in the Ganga-Ghaghara interfluves being deeper. The entire region has suffered great down warping due to the Himalayan upheaval.
The basal rocks seem to have faulted transversely at places and dislocations in the earth's crust along such pre-existing faults or cracks cause earthquakes (Hari Narain, 1965). Fault lines can be seen running N-S or NE-SW, one through west of Muzaffarpur and Patna and the other east of Purnea.
The alluvial deposits have less Kankar formation owing to more riverain character of the plain
and higher incidence of Khadar lands. The region is a homogeneous and featureless plain where monotony is broken by local eminences like river levees and bluffs, sandy features like the Dhus of the Saryupar, the oxbow lakes, Tals and marshy lands (Chaurs) of north Bihar, the badlands and ravines of the Sai and the Gomati rivers and low lands (Jala or Tal) of south Bihar. Besides Ganga, Gomati, Ghaghara, Rapti, Gandak, Kosi (in the north), and Son (in the south) are other important rivers of the region. These rivers very often change their courses and cause severe floods. The Kosi, called 'sorrow of Bihar' has shifted its course over 120 km in recent times.
The Middle Ganga Plain can be broadly divided into two sub regions: (a) the Ganga Plain North, and (b) the Ganga Plain South. The former is further divided into four micro units: (i) the Ganga- Ghaghara Doab, (ii) the Saryupar Plain, (iii) the Mithila Plain, and (iv) the Kosi Plain. Similarly the Ganga Plain South is sub-divided into (v) the Ganga- Son Divide, (vi) the Magadh Plain, and (vii) the Anga Plain.
The relief features of the entire Saryupar and North Bihar Plain are a series of alluvial cones formed by the main rivers alongwith the inter-cones (with fewer gradients).
There are numerous ox bow lakes, tals, dead or deserted channels of the rivers. In Ganga-Ghaghara Doab region the Bhangar is more extensive. The alluvium deposited in the Ganga Plain South has been brought from the southern uplands and is relatively coarser.
It also lacks lakes and tals except in Patna district. East of the Son in the Magadh-Anga. Plain, there are a number of hills running in SW-NE direction.
(c) The Lower Ganga Plain-The Lower Ganga Plain (21° 25'-26° 50'N and 86° 30'-89° 58'E) incorporating an area of 80,968 sq. km., extends from the foot of the Darjeeling Himalaya in the north to the Bay of Bengal in the south (maximum length about 580 km) and from the eastern margin of the Chotanagpur Plateau in the west to the borders of Bangladesh and Assam in the east (maximum width about 200 km).
Its western boundary roughly corresponds with 150 m contour, while Indo-Bangladesh international border forms the eastern limit. It includes the Kishanganj Tahsil of Purnia district (Bihar) and whole of West Bengal (excluding the Purulia and Darjeeling districts).
Though the whole of this plain is now perceived as deltaic, the real delta constitutes about two-thirds of this Plain lying to the south of the Rajmahal-Garo alignment and even continuing into Bangladesh. 'Ganga delta is probably the largest delta in the world.' (Spate and Learmonth, 1967, p. 571).
The eastern part of the Plain is drained by the ri vers (Kartoya, Tista, Jaldhakia, Torsa, Sankosh) joining the Brahmaputra, and the western part by the tributaries (Mahananda, Purnabhaba, Ajay, Damodar, Dwarkeswar, Rupnarayan) of Ganga (Padma-Bhagirathi). In the extreme south-west Kasai and Subarnarekha are other rivers. The general slope of the region is towards south-east and in the delta proper the gradient is less than 2 cm per km.
The Lower Ganga Plain has been formed by the down warping of a part of the peninsular land- mass (between Rajmahal hills and Meghalaya Plateau) and its subsequent sedimentation by the Ganga and the Brahmaputra river systems (which came into being by the disintegration of the Indobrahma system).
The average height of the region above sea level is 50 m which reaches to a maximum of above 150 m in the extreme north (Darjeeling hills) and south-west (Rajmahal hills) and descends to merely 6 m near Kolkata and 3 m near Sagar Island.
The plain has a monotonous surface dissected frequently by the channels of the main streams and their tributaries. On detailed study there are four areas where the relative relief is somewhat significant (about 12 m to 30 m). These include: (i) the Malda West Dinajpur tract showing inliers of the lateritic alluvium, (ii) the tract bordering the Chotanagpur Highlands where the Gondawanas and the lateritic alluvium are found, (iii) the Midnapur Coast consisting of sand dune terraces, and (iv) the Duars (Tarai) of Jalpaiguri and Darjeeling, comprising the submontane tract. Elsewhere the bils (lakes), swamps, marshes and the levees are the only components of the physical landscape.
Physiographically the region can be subdivided into : (1) North Bengal Plain consisting of (a) Duars, and (b) Barind Tract, (2) Delta Proper with (c) Moribund, (d) Mature, and (e) Active deltas, and (3) the Rarh Plain divided into (0 Birbhum-Asansol Rarh, (g) Bankura Rarh, and (h) Midnapur Rarh.
The unassorted materials and older alluvium (Lateritic) constitute the surface of the North Bengal Plain. In Barind Plain, a tract of older alluvium, the important streams like Kosi, Mahananda, Tista and Karatoya etc. make frequent swings in the area.
The Delta Proper is a relatively lowlying region wherein Bils, swamps, Chars, levees, Dangas and coastal dunes are the only features of physical prominence. Here Moribund Delta lies in Murshidabad and Nadia a district which presents evidence of eastward shift of the Ganga river.
It is an area of dead and decaying rivers, The Mature Delta is a land of choked rivers lying to the west of the Bhagirathi-Hugli. Similarly the Active Delta lying in the south-eastern corner is the land of marshes, levees, saline water lakes and mangrove forests (Sundarbans). It has extremely gentle slope and even a 7 m high tide is sufficient to submerge the region upto Kolkata.
The Rarh Plain presents evidence of changing sea level; the old coast line lying 11 km inland and the present Digha Beach showing flat and straight shore line. It is a lateritic region whose southern parts have four parallel lines of dunes (Niyogi and Chakravarti, 1967, p. 205).
(d) The Brahmaputra Plains-The Brahmaputra Plains, also called as Assam Valley, (25°44' - 27°55' N and 89°41' - 96°02' E) are the easternmost part of the Great Plains drained by the Brahmaputra and its tributaries.
These plains from Sadiya (in the east) to Dhubri (near Bangladesh border in the west) are about 720 km long and about 80 km wide covering a total area of about 56,274 sq. km. The region is surrounded by high mountains on all sides, except on the west, and is demarcated by 150 m contour.
Geologically the plain has been formed by the deposition of alluvium, 1500 m thick, upon a sag originated during the period of the Himalayan upheaval. It is very well defined between the Boundary Fault in the north and the Naga Thrust in the south.
The general altitude of the valley ranges from 130 m in the east to 30 m in the west (Sadiya 130 m, Dibrugarh 105 m, Sibsagar 97 m, Jorhat 87 m, Tezpur 79 m, Guwahati 55 m, and Dhubri 34 m) with an average slope of 12 cm per km. It has general slope towards south-west draining into the Bay of Bengal.
The Assam valley is characterised by steep slope along its northern margin but the southern side has a gradual fall from the Meghalaya Plateau. Due to low gradient the Brahmaputra is highly braided river and has numerous riverain islands. Majuli (area 929 km2) is the largest river island in the world.
Another interesting geomorphological feature of the valley is the presence of a number of isolated hillocks or monadnocks on both the banks of the Brahmaputra which have been detached from the Meghalaya Plateau due to the degradational activity of the river.
There is marked difference between the physiography of the north and south banks of the Brahmaputra River. Northern tributaries descending from the Assam Hills form a series of alluvial fans which coalesce and obstruct the courses of the tributaries forcing them to form meanders and adopt parallel course along the main stream (due to the levee of the Brahmaputra).
This has led to the formation of bils, ox-bow lakes, marshy tracts and Terai lands with dense forest cover. The southern part of the valley is less wide and uneven and the tributaries in the south-east are considerably larger.
Here Dhansiri and Kapili through their headword erosion have almost isolated the Mikir and Rengma hills from the Meghalaya Plateau. Here again the eastern part is characterised by meanders and lakes.
The Assam Valley is divided into two sub regions: (1) Upper Assam Valley, and (2) Lower Assam Valley. These are demarcated along 94° E. longitude. The Upper Assam Valley includes the districts of Lakhimpur and Sibsagar and Tezpur Tahsil of Darrang district.
This is a monotonous plain except for the low "hill ranges along the southern and south-eastern borders. The Lower Assam Valley consists of Dhuburi, Goalpara, Barpeta, Kamrup, Nagaon and Darrang (only Mangaldoi Tahsil) districts.
This region does not possess monotonous physiographic characteristics since its landscape is interspersed with spurs of the Meghalaya Plateau. Here right bank tributaries form trelis pattern, while the left bank tributaries of the Brahmaputra exhibit dendrite pattern. Swamps and marshes are numerous in the northern region.