Tista Fan an alluvial fan is a body of detrital sediments built up by a river at the base of a mountain front. The alluvial fan takes the form of a low cone, which is concave, upward to the upstream region. Numerous alluvial fans have been developed at the base of the Himalayan Mountains. The Tista alluvial fan is located to the eastern Himalayan Foreland. The fan named according to the river tista since this river has formed it. The river Tista originates from the Tista-source glacier or Paununri glacier near Khangchung Lake about 7,128m above sea level within the eastern Himalayas in Sikkim of India. After a long journey through the mountainous region it developed an large and extensive alluvial fan that is situated both in India and Bangladesh. The fan apex is located in Darjeeling of West Bengal at an elevation of about 250m above the sea level. The lower middle and lower part of the fan is located in Bangladesh. In the country the fan physically occupies some areas of Rangpur, Dinajpur, Panchgarh, and Thakurgaon districts. The fan is extended southward up to the barind tract. The elevation of the fan base is approximately 34m. The river mahananda marks the western boundary and the Tista itself marks the eastern boundary of the fan that gives it a sub-circular shape.
Besides these rivers, Kulic, tangon, punarbhaba, atrai, little jamuna, karatoya and ghaghat are also flowing over this alluvial fan of which some are straight and others are braided in nature. The fan is developed mainly by coarse-grained sand and cross-bedded sand with interlayering pebbly and muddy layers and occasional gravel beds ranging in age from the Quaternary to Recent. The fan surface slopes towards south-east direction at a rate of about 1.30m per 1.60 km in the middle part and about 0.70m in the distal part. [Kazi Matinuddin Ahmed]
See also [[Bengal Basin|bengal basin].