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The ganga and brahmaputra the most fertile in the region because it contains alluvium soils which is best for agriculture. the soil is travel through the river water and get deposited in the side of the river so that area is very best for agriculture.  
The Indo-Gangetic Plain, also known as Indus-Ganga and the North Indian River Plain, is a 255 million hectare (630 million acre) fertile plain encompassing most of northern and eastern India, the most populous parts of Pakistan, and virtually all of Bangladesh.The region is named after the Indus and the Ganga, and encompasses a number of large urban areas.
The Prinsep Ghat which is located on the bank of the Hooghly River in the city of Kolkata, West Bengal
Boatmen row between Barrackpore and Serampore on the Hooghly river. A photograph shot in 2006The Indus-Ganga plain is bound on the north by the Himalayas which feed its numerous rivers and are the source of the fertile alluvium deposited across the region by the two river systems. The southern edge of the plain is marked by the Chota Nagpur Plateau. On the west rises the Iranian Plateau.