principal southern tributary of the Ganges (Ganga) River (Ganges River), rising in Madhya Pradesh state, central India. It flows north past Manpur and then turns northeast. The river cuts through the Kaimur Range and joins the Ganges above Patna, after a 487-mile (784-km) course. The Son valley is geologically almost a continuation of that of the Narmada River to the southwest. It is largely forested and sparsely populated. The valley is bordered by the Kaimur Range to the north and the Chota Nagpur plateau to the south. The river's flow is seasonal, and the Son is unimportant for navigation. Dams have been constructed on some of its tributaries. At Dehri, in Bihar state, are the headworks of the Son canal system.