Which tributary of the Ganga River popularly known as 'Sorrow of Bihar'?
 

One of the perennial rivers of India, the Kosi River is also known as Sapta Kosi, which translates to "seven rivers."  It is referred to be the "Sorrow of Bihar" because the yearly floods disrupt the rural economy by affecting around 21,000 km sq (8,100 sq mi) of fertile agricultural regions.