- The Kosi basin is bounded on the north by the Himalayas, on the east by Mahananda basin, on the west by the Burhi Gandak basin and on the south by the river Ganga.The basin in India Territory extends over areas of districts Saharsa, Purnea, Khagaria, Madhubani, Sitamarhi, Muzaffarpur and Darbhanga in Bihar state. The more important rivers in this basin are kosi (main stem), Kamla Balan Adhwara Group of riversan Bagmati.
- The river Kosi is formed by the confluence of three streams namely the Sun Kosi, the Arun Kosi and Tamur Kosi all taking their origin in the Himalayan region of Nepal and Tibet. After confluence, Kosi flows in narrow gorge for about 10 km and debouches into plains near Chatra. River enters in Indian Territory about 25 km below the Chatra gorge at Hanuman nagar. After that Kosi runs in a sandy track and almost in level plains and finds its way southwards into the River Ganga through a number of channels. The Kosi drains an area of 74,500 sq.km of which only 11,070 sq.km lie within Indian Territory. The Kosi is well known for its tendency to change its course generally in westward direction. During the last 200 years, the river has shifted westwards for a distance of about 112 km and has laid waste large tracks of agricultural land in Darbhanga,Saharsa and Purnea districts. The total catchments area upto its outfall in the River Ganga is 100800 sq.km.
- Apart from the three streams which form the Kosi, a number of tributaries join the river in course of its flows through the plains from the right bank. These are Trijunga, Bhutahi Balan, Kamla Balan and Bagmati.
- The Trijunga originates in the foot hills of Himalayas and join Kosi in Nepal having catchments area of 747 sq.km in the hills.The Bhutahi Balan originates in the foot hills of Himalas and out falls in Kosi a few Km below Nirmali having a catchments area of 1,505 sq.km.
- The River Kamala originates in the Mahabharat range of hills in Nepal at an elevation of 1,200 mt (Latitude 27°15”E). During the course in Nepal, the Kamla receive a number of tributaries like the Chandaha, the Thakur and the Jawakhota. After flowing about 50 km in Nepal the river enters Indian territory at Begraha above Jai Nagar in Madhubani district. It is joined by Souri,Dhewri and Balan. The river thereafter known as Kamla Balan. It flows in southeasterly dfirection and out falls into Kosi near Darjia. The river has a catchments of 2,980 sq.km and length 328 km.
- Adhwara group of river originates from the foot hills of Nepal. The vast tract of very fertile land in North Bihar lying between river Kamla and Bagmati is known as the Adhwara Sub-Basin. This basin is traversed by a network of river namely Adhwara, Jamura, Sikau, Burhand, Khiroi, Singhi, Marha, Rato Dhaus and Darbhanga Bagmati. After traversing the whole basin inter connected finaly form two distinct drainage channels namly Khiroi and Darbhanga Bagmati. These two also meet just above Ekmighat road bridge. The combined channels flowing southward falls into the river Bagmati just above Hayaghat Railway Bridge. The total catchments area of these rivers is 4,962 sq.km,out of which 2,597 sq.km is in India.
- River Bagmati originates from Shivpuri range of hills in Nepal at an elevation of 1,500 m. at north Latitude 27°47” and east Longitude 85°17” flowing in western directrion the river cuts across the Mahabharat range of hills and enters India at about 2.5 kms. upstream of Dheng Bridge in districts Sitamarhi. It receives tributaries Lakhandai, Lalbakya and Adhwara system. The river ultimately out falls in Kosi at upstream of Baltara in Khagaria district. Its total catchments area is 13,424 sq.km out of which 6,320 Sq.km lie in India. Its total length is 589 km.