Bihar is India’s most flood- prone State, with 76 percent of the population, in the north Bihar living under the recurring threat of flood devastation. About 68800 sq Km out of total geographical area of 94163 sq Km comprising 73.06 percent is flood affected.
The plains of Bihar, adjoining Nepal, are drained by a number of rivers that have their catchments in the steep and geologically nascent Himalayas. Kosi, Gandak,BurhiGandak, Bagmati, KamlaBalan, Mahananda and Adhwara Group of rivers originates in Nepal, carry high discharge and very high sediment load and drops it down in the plains of Bihar. About 65% of catchments area of these rivers falls in Nepal/Tibet and only 35% of catchments area lies in Bihar. A review by Kale (1997) indicated that the plains of north Bihar have recorded the highest number of floods during the last 30years. In the years 1978, 1987, 1998, 2004 and 2007 Bihar witnessed high magnitudes of flood. The total area affected by floods has also increased during these years. Flood of 2004 demonstrates the severity of flood problem when a vast area of 23490 Sq Km was badly affected by the floods of Bagmati, Kamla&Adhwara groups of rivers causing loss of about 800 human lives, even when Ganga, the master drain was flowing low.
Bihar witnessed one of the most disastrous floods in the history of Bihar in the year 2008, an impoverished and densely populated state in India. A breach in theKosi embankment near the Indo-Nepal border (at Kusaha VDC, Sunsari district, Nepal) occurred on 18 August 2008. The river changed course and inundated areas which hadn't experienced floods in many decades. The flood affected over 2.3 million people in the northern part of Bihar.
The 2013 Flood in Bihar affected more than 5.9 million people in 3,768 villages in 20 districts of the state.