The Terai is crossed by the large perennial Himalayan rivers Yamuna, Ganges, Sarda, Karnali, Narayani and Kosi that have each built alluvial fans covering thousands of square kilometres below their exits from the hills.
The reduction in slope as rivers exit the hills and then transition from the sloping Bhabhar to the nearly level Terai causes current to slow and the heavy sediment load to fall out of suspension.