The Ganges Plain lies between the northern edge of the Deccan, which plunges abruptly into the alluvial plain. It is one of the largest plains in the world and stretches almost horizontally for a thousand miles from Amritsar to Calcutta. The whole area is dominated by the River Ganges itself which forms a natural demarcation line between the differing northern and southern parts of the valley. There is notable climate variation from east to west. Below is given a brief note on the natural features of the Ganges watershed and its climatic variations.
The Ganges Watershed
The River Ganges, rising in the Himalayas at a height of about 14,000 feet, is nearly 2,000 miles long and has a drainage basin of 350,000 square miles. At first it is a mountain torrent, but when it enters the plain at Haridwar the height is only 1,000 feet, and it still has more than 1,500 miles to flow. It flows rather at a slow pace across the plain broad and deep with a good run-off throughout the year round. There is marked rising of the river during the summer monsoon due to both rainfall and melting snow. During the dry season the river is sunk into its flood bed, which is often two or three miles wide.
Left-Bank Tributaries
Most of the left-bank tributaries flow at first almost parallel to the main river which they join farther downstream. These tributaries fed by Himalayan torrents have worn courses for themselves in the unconsolidated sedimentary rocks, and are fairly regular except for a few violent floods. The Gomati, which does not rise in the Himalayas, flows in fairly mature meanders. Kosi rises on the slopes of Everest and flows straight down an alluvial fan on which it frequently changes course since Bihar is a region of heavy rainfall. Besides the Kosi is continually spreading micaceous sand over the plain making useless fields that were formerly fertile and productive.
Right-Bank Tributaries
The Yamuna, a right-bank tributary which originates in the Himalayas, is joined by rivers from the northern edge of the Deccan carrying little alluvium. Because of this the Yamuna which has a regular flow discharges into the Ganges water much clearer than of the main stream. Another right-bank tributaries is the Son, which meets the Ganges near Patna. After Patna the Ganges is both wide and great in volume, and at its confluence with the Brahmaputra it has a flow of 600,000 cubic feet per second. Before entering Bengal the river meets the obstacle of the Rajmahal Hills and the southern part of the river valley, which is often narrow disappears completely.
The Transition of Climate and Region
It is very difficult to define the east to west divisions of the Ganges Plain for usually the change is imperceptible. The most striking difference is that of climate, with a marked drought and range of temperature in Rajasthan, and a high rainfall and equable temperature in Bengal. A more acceptable division of the Ganges Plain is into rice-and-wheat-growing areas. The wheat-growing region of the Upper Ganges extends roughly as far as Allahabad and the rice-growing area of the Middle Ganges as far as Bengal. The Delta is also primarily a rice-growing area.