The most devastating feature of Kosi, Bihar's sorrow, has been its ruthless course shifts. In the last 200 years, it has moved westwards by as much as 110 km averaging some 0.5 km every year. Up to the beginning of the 12th century BC, it was flowing through the eastern part of Purnia and Katihar. In the beginning of the 1930s, it made a big westward thrust and passed through Saharsa, Madhubani, Darbhanga, Samastipur, Khagaria, Medhapura and Katihar districts of the state, devouring other, smaller, rivers such as Tilyuga and Bhutan! Balan. By 1952, Kosi was flowing through Saharsa Madhubani-Darbhanga region. By the time the L N Mishra-led government began working on the embankments, the river was wrecking havoc in an area of around 5,000 sq km. Why does Kosi change its course? Experts point out that through a continuous process of deposition of silt, the river bed rises considerably, almost "pushing" Kosi on to a new course.

Rising in the Himalayas at a height of some 5,400 m, Kosi flows through Tibet, Nepal and India, covering some 725 km before merging into the Ganga at Kursela. It covers an area of 74,500 sq km, including 5,704 sq km under the glaciers in the Himalayas. Kosi enters Bihar through Nepal and passes through Saharsa, Darbhanga and Purnia in the state, and carries a silt load of 92,000 cubic feet.