The Northern Plains is a transition zone between the Northern Mountain belt and the Peninsular Plateaus of India. It is the largest alluvial tract in the world, extending for about 3200 km from the mouth of the Indus River in the west up to the mouth of the Ganga River in the east. The average width is about 150 km to 300 km.
The northern boundary of the Northern Plains is well-defined by the foothills of the Northern Mountain belt, but the southern boundary is irregular along the northern edge of the Peninsular Plateau.
The Northern Plains have been formed by the filling up of the deep depression lying between the Northern Mountains and the Peninsular Plateau. It was done by the Ganga and the Brahmaputra rivers along with their numerous tributaries, coming from the two landmasses on either side. The deposits include a thick layer of sand, clay, loam and silt.
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The Northern Plains have been an area of great significance in the Indian history. The fertile alluvial plains all over the world have supported civilizations, due to the rich soil, suitable climate and adequate water supply.
The Northern Plains are remarkably homogeneous with little variation in relief features for hundreds of kilometres. But they have their own diversities and the monotony of the physical landscape has been broken by a few significant features, such as bhabar, terai, bhangar and khadar .
On the basis of regional variations, the Northern Plains can be divided into the following regions.
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1. The Rajasthan Plain,
2. The Punjab-Haryana Plains
3. The Ganga Plain, and
4. The Brahmaputra Plain
1. The Rajasthan Plain:
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The Rajasthan Plain is located towards the west of the Aravali Range and the region extends for about 640 km, with an average width of about 300 km. It covers western Rajasthan and about two-thirds of this region is desert. It is about 300 metres above the mean sea level. In general, the eastern part of the desert is rocky, while the western part has shifting sand dunes.
The Rajasthan Plain is drained by a number of seasonal streams originating from the Aravali ranges. Luni is an important inland river of this region. It flows into the Rann of Kachchh. North of Luni, there is a large area of inland drainage. It has several dry river beds. It is located on the eastern edge of the Thar desert having several saline lakes. The largest is the Sambhar Lake, which is located about 65 km west of Jaipur.
The general slope of the Rajasthan Plain is from east to west, i.e. towards the Indus River. But the southeastern part of the plain slopes towards the Rann of Kachchh.
2. The Punjab-Haryana Plains:
Towards the northeast of the Great Indian Desert lie the fertile plains of Punjab and Haryana. This plain extends for about 640 km from the northeast to the southwest and about 300 km from west to east. The average height of the plain varies from about 300 metres in the north to about 200 metres in the southeast. In the east, the Delhi ridge separates it from the Ganga Plain.
The Punjab-Haryana plain owes its origin to the depositional activities of the Satluj, Beas and Ravi rivers. The southeastern part of the plain, bordering the Rajasthan Plain, is sandy and has shifting sand dunes. The Punjab Plain is made up of doabs—the land between two rivers.
The area between Ghaggar and the Yamuna rivers lies in Haryana and forms the Haryana Plain. It acts as a water-divide between the Yamuna and the Satluj rivers.
3. The Ganga Plain:
The Ganga Plain is the largest part and extends from the Yamuna River in the west up to Bangladesh in the east, covering a distance of about 1500 km with an average width of about 300 km. It covers the states of Uttar Pradesh, Bihar and West Bengal.
The Ganga along with its large number of tributaries, such as Ramganga, Gomti, Ghaghara, Gandak, Kosi, Yamuna, etc., from the north and Son, Chambal, Betwa, Kosi and others from the south, have brought large quantities of sand and silt from the mountains and plateaus respectively, and deposited in this vast plain. The general slope of the entire Ganga Plain is towards the east and the southeast. The average elevation of the plain is about 200 m above the sea level.
In the western part of this plain lies the Ganga- Yamuna Doab. Towards the east of the Doab is the low-lying Rohilkhand.
In the middle part, the flow of the rivers is sluggish and most of them keep shifting their courses. This has made the region prone to frequent floods. The Kosi River, known as the ‘Sorrow of Bihar’ has shifted its course by about 100 km in the recent times.
In the lower part, the Ganga and the Brahmaputra rivers divide itself into several channels in this region to form the largest delta in the world.
The lower part, of the delta, called the Sundarbans, is covered with thick tidal and mangrove forests. The sea-facing region of the delta has a large number of estuaries, mangrove swamps, sandbanks and islands.
4. The Brahmaputra Plain:
The easternmost part of the Northern Plains is drained by the Brahmaputra river and its numerous tributaries. The Brahmaputra River originates in Tibet and is locally known as Tsangpo (the Purifiers). Before entering India, it cuts through the Dihang gorge and enters the Assam Valley. This plain is about 720 km long and about 60-100 km wide. The general slope is from the northeast to the southwest. The region is surrounded by high mountains except on the west.
A large number of tributaries coming from the Assam hills in the north join the main river and form alluvial fans. There are large marshy tracts in this area. The alluvial fans have led to the formation of terai or semi-terai conditions.