In Bihar, particularly in the region south of river Ganga, the rainfall has been inadequate and erratic leading to frequent recurrence of severe famines. Following are the main canal systems of the state :
1. Son Canal-A barrage has been built across the Son river neat Dehri from where two canals have been taken out. The eastern canal taking off at Barun was completed in 1875. This 130 km long canal is called Patna canal which irrigates about 3 lakh hectares of land in Patna, Gaya and Aurangabad districts. The western canal originates from Dehri and feeds the Ara, the Buxar and Chausa branches. It irrigates 3.5 lakh hectares of land in Ara, Shahabad and Bhojpur districts.
2. Kosi Project Canal-under the project a barrage has been constructed across the Kosi River near Hanuman Nagar from which two canals would be taken out to irrigate about 8.73 lakh hectares of cropped area. The eastern Kosi canal is complete. Its total length is 127 km consisting of Muraliganj, Janakinagar, Banamanakhi and Agariy branches. It irrigates about 4.34 lakh ha of land in Purnea, Muzaffarpur, Munger, Darbhanga and Saharsa districts. The western Kosi canal has a length of 112.65 km (including 35.2 km in Nepal). On completion it would irrigate about 3.14 lakh hectares of land area.
3. Gandak Canal-Under the project a long (740 m) barrage has been built across Gandak river near Triveni Ghat from which a number of canals have been taken out to irrigate about 14.58 lakh hectares of land in Nepal, Bihar (Champaran, Saran, Chhapra, Muzaffarpur, Darbhanga, Siwan, Vaishali (Bihar), Gorakhpur and Deoria (Uttar Pradesh) districts. The Saran (66 km), the Tirahut (257 km), Dun (95 km), Nepal east (80.47 km) is the important branches of the canal.