Tista Floodplain Bangladesh is mainly occupied by the Recent alluvial and floodplain sediments. Tista floodplain covers most of Rangpur and adjoining regions that actually lies in the younger part of Tista alluvial fan that covers several different landscapes. Physically, the floodplain stretches between the Himalayan Piedmont Plain in the west and the right-bank of the north-south flowing brahmaputra in the east. The diversity results from the fact that the tista has occupied and abandoned several different channels during the past few thousand years including the valleys now occupied by mahananda, punarbhaba, atrai, little jamuna, karatoya and ghaghat rivers. The small floodplains of dudhkumar and Gangadhar rivers are also included in this unit. The main geomorphic agent of this unit is river Tista. This river along with the others brings sediments of different sizes to the floodplain at different times.
monsoon climate dominates this region where rainfall is abundant. The average annual rainfall in this region is little over 1,900 mm. Tista is the prominent river of this region, which has a mean monthly discharge of about 2,430 cumec. Tista rises in Chitamu Lake in Tibet that joins the Brahmaputra in the Rangpur district of the country. It flows through a magnificent gorge known as the Sivok Gola in Darjeeling district. It is a wild river in the Darjeeling Hills where its valley is clothed with dense forest, but its drainage area in the mountain is only 12,500 sq km. Up to the close of the 18th century it flowed into the ganges, but after the destructive floods of 1787 in which a large part of the Rangpur district was laid waste, it suddenly turned east and joined Brahmaputra. The monthly mean discharge of the dharla that borders the western boundary of the floodplain is about 1,440 cumec. There are many old channels that used to be occupied by this river. Karatoya through which it joined the Ganges is still known as the Buri Tista or Old Tista. [Kazi Matinuddin Ahmed]
See also physiography.