Why is river kosi sorrow of bihar?

The Kosi River that flows from Nepal to India was known as the " Sorrow of Bihar", because it had caused widespread human suffering in the past & at present. the embankments built on both sides of the Kosi few decades back have temporarily helped to control this river. It is feared that very soon the Kosi would abandon its present course triggering off a new cycle of damages and destructions. After such incident, the Kosi might not anymore be only the sorrow of Bihar. It could be the sorrow of the West Bengal and Bangladesh apart from the terrible havoc the Kosi floods could be playing with the safety of people in southeastern Nepal.