Kosi is a major tributary of the Ganga River which originates at an altitude of 7,000 m in the Himalayas.  The total drainage area of the Kosi River is 74,500 sq. km out of which 11,000 sq. km lie in India. The Kosi basin lies between 85° and 89° east longitudes and 25° 20’ and 29° north latitudes. Nearly 80% of the Kosi catchment is in Nepal and Tibet. About 77% of the area is under cultivation.

The total catchment area of Kosi basin is 95,156 sq km of which 20,376 sq km lies in India. Kamla Balan and Bagamati are two major tributaries on the right side of the river in Bihar.