<P> The Kosi alluvial fan is one of the largest in the world, and extends from Barāhkṣetra across Nepalese territory, covering northeast Bihar and eastern Mithila to the Ganges, 180 km (110 mi) long and 150 km (93 mi) wide . It shows evidence of lateral channel shifting exceeding 120 km (75 mi) during the past 250 years, via at least twelve major channels . The river, which flowed near Purnea in the 18th century, now flows west of Saharsa . A satellite image shows old channels with a confluence before 1731 with the Mahananda River north of Lava . </P> <P> The Kosi River is known as the "Sorrow of Bihar" as the annual floods affect about 21,000 km (8,100 sq mi) of fertile agricultural lands thereby disturbing the rural economy . </P> <P> The Koshi has an average water flow (discharge) of 2,166 cubic metres per second (76,500 cu ft / s). During floods, it increases to as much as 18 times the average . The greatest recorded flood was 24,200 m / s (850,000 cu ft / s) on 24 August 1954 . The Kosi Barrage has been designed for a peak flood of 27,014 m / s (954,000 cu ft / s) (2). </P> <P> Extensive soil erosion and landslides in its upper catchment have produced a silt yield of about 19 m / ha / year (10 cu yd / acre / yr), one of the highest in the world . Of major tributaries, the Arun brings the greatest amount of coarse silt in proportion to its total sediment load . The river transports sediment down the steep gradients and narrow gorges in the mountains and foothills where the gradient is at least ten metres per km . On the plains beyond Chatra, the gradient falls below one metre per km to as little as 6 cm per km as the river approaches the Ganges . Current slows and the sediment load settles out of the water and is deposited on an immense alluvial fan that has grown to an area of about 15 000 km . This fan extends some 180 km from its apex where it leaves the foothills, across the international border into Bihar state and on to the Ganges . The river has numerous interlacing channels that shift laterally over the fan from time to time . Without channelisation, floods spread out very widely . The record flow of 24 200 m / s is equivalent to water a metre deep and more than 24 km wide, flowing at one metre per second . </P>

Which river has the smallest basin in india