Q. Which river is NOT a tributary of the northern Koel River?
Answer: Ajay
Notes: The North Koel River originates in Jharkhand's Ranchi Plateau and joins the Son River. Auranga and Amanat are its main tributaries flowing through Palamu division. Ajay River flows through West Bengal and is a tributary of the Bhagirathi-Hooghly river system. Ajay is not connected to the North Koel or Son river basins. Hence, Ajay is not a tributary of the northern Koel River.