Answer: Kosi
Notes: Kosi river changes its course very frequently in Bihar. It is also known as the "Sorrow of Bihar". It carries huge amount of water and sediments and when it enters plains then it changes its course very frequently and regularly. Due to the frequent changes in the direction and course, huge amounts of sediments get deposit in very less time which leads to overflow of water causing devastating floods in Bihar, specially, in Monsoons.
