Ganga Plain: Location, Formation and Regions | India | Geography

In this article we will discuss about:- 1. Location of Ganga Plain 2. Geological Formation of Ganga Plain 3. Physiographic Regions.

Location of Ganga Plain:

The Ganga plain is a major part of the Great Plains of north India which comprise the Rajasthan plain, the Punjab plain and the Ganga plain and have been formed through the process of alluviation by the Satluj, the Ganga and the Brahmaputra drainage sys­tems during late Tertiary and Quaternary periods.

In fact, the great plains are in the form of a transitional belt between the Himalayas in the north and the Deccan plateau in the south covering an area of more than 7,77,000 km² from Rajasthan to West Bengal. The Great Plains run for a distance of 2400 km from west to east and have a width of480 km in the west and 144 km in the east.

Geological Formation of Ganga Plain:

Lithologically, the Ganga plain is characterized by Quaternary alluvia with varying combinations of clay, silt, sand and gravels. It is, thus, evident that lithologically the Ganga plain is monotonous except some variations in the northern part where the Bhabars of the Himalayas grade into the plains and in the southern part.

In fact, ‘alluvium is one continuous and conformable series of fluviatile and subaerial deposits, mainly composed of unconsolidated beds of clay, sand, gravel, and their mixture in varying proportion’. The basement of alluvial depos­its and their exact thickness could not be ascertained inspite of several attempts.

According to Oldham the thickness of alluvium ranges between 4000-6000m whereas Glennie’s estimate of alluvium depth comes to about 2000m. Ganga alluvia are classified into khadar and bhangar. Khadar represents alluvial depos­its in the riverine tracts or the flood plains of the rivers where the constituents of khadar i.e., silt, caly and sand, are renewed almost every year due to alluviation fol­lowed by floods.

On the other hand, bhangar represents older alluvium of relatively higher lands which are not submerged under water during floods. The Bhabar and Tarai regions contain gravel deposits comprising coarse sands, pebbles and cobbles with clay and silt.

‘One distinctive character of the bhangar is the formation of kankarpans (hardpans) in the sub-soil zone through capillary action owing to the alternating calcareous sand and clay beds here as also elsewhere in the zone of seasonal rainfall, which adds to soil moisture reten­tion in the subsoil zone’.

D.C. Dassarma and S. Biswas (1971-72) estab­lished the following stratigraphic successions of the Quaternary alluvium deposits resting over the Vindhyan basement in the alluvial-filled Belan basin (Allahabad district, U.P.) from below upward:

(i) Gravel-mottled clay formation,

(ii) Red brown gravel-sand formation,

(iii) Yellow silt formation,

(iv) Caliche formation,

(v) Buried soil layer,

(vi) Aeolian sand formation, and

(vii) Modern soil horizon.

The sequence of older and newer alluvia is repeated throughout upper, middle and lower Ganga plains but ‘there is conspicuous absence of older allu­vium in the delta region, which is quite extensive in the northern parts of the region’.

Physiographic Regions of Ganga Plain:

Geomorphologically, the entire Ganga plain is monotonous as no significant bold reliefs of mountain­ous and dissected plateau regions are found. Extensive flood plains, natural levees, sharp meanders, ox-box lakes, gullied riparian tracts, braided channels, bluffs etc. are the only morphological features of the Ganga plain.

Though the entire Ganga plain exhibits geomorphological homogeneity but it is divided into three regions following R.L. Singh (1971) viz.:

(1) Upper Ganga plain,

(2) Middle Ganga plain, and

(3) Lower Ganga plain.

(1) Upper Ganga Plain:

Upper Ganga Plain lies between the Yamuna valley in the west and 100m contour in the east cover­ing an area of 1,49, 029 km² of Uttar Pradesh. The drainage of the area comprises two drainage systems of the Ganga and the Yamuna (It may be mentioned that the Yamuna is also a tributary of the Ganga and hence the upper Ganga plain is drained by a single drainage system i.e. the Ganga drainage system).

Most of the streams adopt parallel courses and join the master streams at acute angles. Extremely low channel gradi­ent because of almost level plain country makes the rivers sluggish and their erosive power is markedly reduced. The drainage pattern is pinnate but the overall pattern is dendritic. Significant rivers are the Yamuna, the Ganga, the Ramganga, the Gomati, the Ghaghra and their numerous tributaries.

Though linear erosion is very limited but the areal erosion in the form of soil erosion is more wide­spread in the Ganga plain. Soil erosion is the most common widespread geo-environmental problem of the major catchment areas of big rivers in Uttar Pradesh, Micropedogenic erosional processes are the primary and basic mechanism responsible for the soil loss.

During the rainy season rill and inter-rill erosion causes soil loss from the bhangar (older alluvial areas) and the khadar (new alluvial area) and the eroded sediments are brought to the main rivers via rills, gullies, nalas, and tributary streams which in due course partly deposit them in the flood plains and partly entrain the sediments from Uttar Pradesh (upper Ganga plain) to Bihar (mid­dle Ganga plain).

Foothill zones of the Himalayas (in the north) and the foreland of the Indian Peninsula (in the south) and riparian tracts of major alluvial rivers are the main sites of active and rapid rate of fluvial erosion whereas vast alluvial tract is washed out through surface runoff. Rill and gully erosion is responsible for acceler­ated rate of soil erosion and increase in sediment dis­charge and sediment load factor in the major rivers’.

‘Total amount of average annual silt carried away from Uttar Pradesh to Bihar by the Ganga system (including the Yamuna system) is about 23,456 ha meters or 328.384 million tonnes and average annual runoff of the Ganga system at U.P.-Bihar border is 21,328 thousand ha meters or 213.38 thousand million m³.

The average annual silt load factor is about 3.476 ha m/100 km²/year. It means that every year about 3.476 mm of upper soil of Uttar Pradesh (upper Ganga plain) is eroded and carried away to the state of Bihar (middle Ganga plain) by the Ganga river system’.

The topographic features include alluvial fans and cones in the submontane or piedmont zone to the north of plains, river bluffs, natural levees, flood plains, meanders, meander cut-off, ox-bow lakes, uplands or bhangar lands, sandy stretches or ‘bhurs’, gullied riparian tracts of major and tributary streams, broken river banks, braided channels, micro-sea-sonal forms on river beds (e.g. sand bars, sand islands, shoals etc.), dense network of gullies and badland in the Yantuna- lower Chambal tract and in the intervening zones between the Ganga plain and the foot-hill zone of the foreland of Indian Peninsula etc.

‘Thus, the micro-level topographic facets and their regional characteristics render possible delinea­tion of as many as four physiographic units i.e.:

(i) The submantane belt,

(ii) The Ganga-Ghaghra doab region,

(iii) The Ganga-Yamuna doab, and

(iv) Yamunapar in the ravine tract’.

(2) Middle Ganga Plain:

Middle Ganga Plain covering an area of 1,44,409 km² is spread over Eastern Uttar Pradesh and whole of Bihar plains. ‘The western and eastern sides of the region are wide open forming as it does, the central part of the east-west continuum of the vast isotropic Ganga Plain-there is no physical boundary as the plain imperceptibly opens up in the west from out of the Upper Ganga plain and so invisibly dies out into the lower Ganga plain in the east.

As such, it is a transitional region, par excellence, interposed in the enormity of the Ganga valley, and all its physical, human and economic fabric seems to have been woven out of the various unique phenomena interplaying in transition’ .

The region is drained by the Ganga and its major tributaries like the Ghaghra, the Gandak, the Kosi and their numerous tributaries in the north middle Ganga plain and the Son, the Punpun, the Mohani etc. drain the south middle Ganga plain. Geologically, the region has more or less the same alluvial formations as those of the upper Ganga plain.

The rivers have developed highly sinuous and meandering courses and they are notorious for furious floods and frequent changes in their courses. The Kosi River has registered continuous westward shifting in its course but now the channel has been stabilized by artificial levees (embankments). Similarly, there have been significant changes in the channels of the Ganga, the Ghaghra, the Son etc.

The Ganga channel becomes braided during post-monsoon period and the river bed morphology comprises nu­merous riffles, pools, braids, sand bars, sand islands, sand flats and seasonal bed sand dunes. The north middle Ganga plain is characterized by the presence of numerous palaeochannels, ox-bow lakes, tanks and ponds locally known as tals (formed due to shifting in river courses) which are of very rare occurrence in the south middle Ganga plain. Other morphological fea­tures include natural levees, ravines and badlands in the riparian tracts of the rivers, flood plains, meander loops, eroded but unstable river banks, sandy features like dhus etc.

‘Obviously, it is difficult to divide the region into physical sub-units on any prominent foundation of reliefs, except through the help of the river systems which generally carve out somewhat inter- distin­guishable relief and slope, differential nature of drain­age based on rainfall regimes and the proximity of the hills, and the all-resultant sub-soil water table of vary­ing depths in different parts of the region’.

Thus, following R.L. Singh (1971) the Middle Ganga Plain is divided into two broad subregions viz.:

(A) the Ganga Plain North, and

(B) the Ganga Plain South.

The Ganga plain North is further subdivided into:

(i) The Ganga-Ghaghra Doab,

(ii) The Saryupar Plain (the Ghaghra-Gandak interfluve),

(iii) The Mithila Plain (the Gnadak-Kosi interfluve), and

(iv) The Kosi Plain (the Kosi-Mahananda interfluve).

While the Ganga Plain South is subdivided into:

(i) The Ganga-Son di­vide,

(ii) The Magadh Plain, and

(iii) The Anga Plain.

(3) Lower Ganga Plain:

‘The Lower Ganga Plain, in real sense of the term, includes the Kishanganj tahsil of Purnea district (Bihar), whole of the West Bengal State (excluding the Purulia district and the mountain­ous parts of Darjeeling district) and most of East Pakistan (now Bangladesh) as well. Though the whole of this plain is now perceived as deltaic, the real delta constitutes about two-third of this plain lying to the south of the Rajmahal-Garo alignment….The region embraces an area of about 80,968 km², extending from the foot of the Darjeeling Himalaya in the north to the Bay of Bengal in the south (maximum stretch about 580 km) and from the edge of the Chotanagpur High­lands, in the west to the border of East Pakistan (Bangladesh) and Assam in the east (maximum stretch about 200 km)’.

The region is drained by the streams of two major drainage systems of the Ganga and the Brahmaputra and a few minor systems viz. the Kasai system, the Subarnarekha system, the Mahananda sys­tem, the Karatoya system, the Tista system, the Sankosh system, etc. The Rarh Plain is drained by the Mayurakshi, the Damodar, the Dwarkeshwar, the Kasai, the Subarnarekha etc.

Geomorphologically, the region, like the upper and middle Ganga Plains, is monotonous as the entire region is almost featureless plain except a few local reliefs of 10 to 30 m rising above the general surface. Malda-West Dinajpur tract, the tracts border­ing the Chotanagpur plateau, coastal areas of Midnapur, and duars (equivalent to tarai of Uttar Pradesh) of Jalpaiguri and Darjeeling are the areas where some reliefs are noted.

For example, lateritic alluvium in the Malda-Dinajpur tract has been dissected to produce prominent reliefs; the bordering areas of the Chotanagpur have been gullied and ravinated; coastal areas of Midnapur are characterized by a series of sand dunes and the lateritic alluvium has been intensively gullied to form badland; the duars of Jalpaiguri and Darjeeling are characterized by swamps, marshes, bills, levees etc.

The alluvial morphological characteristics of the re­gion have been conditioned by the tectonics, changes in river courses and sea levels. R.L. Singh (1971) divided the Lower Ganga Plain, on the basis of micro- order diversities within the apparently homogenous surface configuration, into the following physiographic subdivisions:

(i) the northern plain characterized by duars (tarai) and Barind tract;

(ii) The Delta proper comprising:

(a) the land of dead and decaying rivers (Moribund delta) in the north (Murshidabad and Nadia),

(b) the active Delta of the Sundrabans and

(c) the mature delta (parts of Birbhum, Burdwan, Midnapur, and entire districts of Hooghly and Howrah);

(iii) the western margin of the delta is called the Rarh plain where the lateritic alluvial landscape along with the coastal scenery at Digha beach has developed.