It extends from the base of the Terai in the north to the Ganga in the south, covering an area of about 56,980 Sq Km. It spreads over the whole of Tirhut, Saran, Darbhanga and Kosi divisions and has a gentle slope towards the south. The Ganga flows from west to east near the southern margin of the plain. Towards the north and north-west in the east and west Champaran districts, the country begins to undulate and the alluvial plain gives place to broken hilly region known as the Dun or Ramnagar Dun. This consists of a range of low hills. Below these hills, large grassy prairies watered by numerous hill streams extend southwards and eastwards. The soil even at the foot of the hills has no rocky formation and whenever water can be impounded, rich growth of crop is possible.

The South Ganga Plain

The alluvial filling south of the Ganga is shallow, a mere veneer and the Peninsular edge is very rugged. Many groups of small craggy hills rise up to 488 meters from islands of bare rock or scrub. In the west, where the stream Sone makes a great deltaic reentrance into the older rocks, this alluvial strip is some 137 Km wide. But in the east where the Rajmahal hills lies on the extreme north-east point of the Peninsula, it goes almost directly on to the Ganga. The river bank itself lies high, except in Bohjpur district and at high water the tributaries are flooded and pushed back. The Punpun valley, parallel to the stream Sone on the east, is thus annually flooded.

Both in the north and the south of Ganga, the construction of railways across the drainage causes local but sometimes disastrous water logging and flooding. Some of these temporary inundations are agriculturally useful, either rabi crops are grown on them when they dry out or they are bunted for producing dry weather rice.

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