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The Son River or Sone of central India is the largest of the Ganges' southern tributaries. The Son originates in Chhattisgarh state, just east of the headwaters of the Narmada River, and flows north-northwest through Madhya Pradesh state before turning sharply eastward when it encounters the southwest-northeast-running Kaimur Range. The Son parallels the Kaimur Range, flowing east-northeast through Uttar Pradesh and Bihar states to join the Ganges just above Patna. Geologically, the lower valley of the Son is an extension of the Narmada Valley, and the Kaimur Range an extension of the Vindhya Range.

The Son river at 784 kilometres (487 miles) long, is one of the largest rivers of India. Its chief tributaries are the Rihand and the Koel. The Son has a steep gradient (35-55 cm per km) with quick run-off and ephemeral regimes, becoming a roaring river with the rain-waters in the catchment area but turning quickly into a fordable stream. The Son, being wide and shallow, leaves disconnected pools of water in the remaining part of the year. The channel of the Son is very wide (about 5km at Dehri) but the floodplain is narrow, only 3 to 5 km wide. In the past, the Son has been notorious for changing course, as is traceable from several old beds on its east. In modern times this tendency has been checked with the anicut at Dehri, and now more so with the Indrapuri Barrage.

Indrapuri Barrage is one of the longest dams in India,^{[citation needed]}, storing a large amount of water. It is located nearly 5 km from Dehri. From it flow 2 major and several other small canals which supply the whole of western and central Bihar with water for irrigation.