The Kosi Megafan: The Best-known Himalayan Megafan

Abstract

The conical-shaped Kosi Megafan in north Bihar Plains (eastern Ganga Plains), eastern India is one of the most cited megafans in the world in the international literature. Named after its principal drainage, the Kosi River, this megafan has a distinct convex-up topography. Numerous palaeochannels on the fan surface represent the remnants of the earlier courses of the Kosi River through its westward journey during the last 200 years. Like other fans in the world, the Kosi megafan has formed due to high sediment flux, fluctuating discharge regime and the availability of large accommodation space. It is believed that the fan building process of the Kosi continues till date and this explains the unstable behavior of the river. The construction of a barrage close to the mountain exit and embankments on both sides of the river have certainly retarded the natural fan-building processes but the Kosi Megafan is still one of the most fascinating landforms on this planet.