The Son river at 784 kilometres (487 mi) long, is one of the largest rivers of India. Its chief tributaries are the Rihand and the North 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 5 km at Dehri on sone) but the floodplain is narrow, only 3 to 5 km wide. In the past, the Son has been notorious for changing course, as it is traceable from several old beds near its east bank. In modern times this tendency has been checked with the anicut at Dehri, and now more so with the Indrapuri Barrage.
In Bihar, this river forms border line between Bhojpuri and Magahi speaking regions.
Sir John Houlton, the British administrator, describes the Son as follows, "After passing the steep escarpments of the Kaimur range, it flows straight across the plain to the Ganges. For much of this distance it is over two miles – and at one point, opposite Tilothu – three miles wide. In the dry weather there is vast expanse of sand, with a stream not more than a hundred yards wide, and the hot west winds pile up the sand on the east bank, making natural embankments. After heavy rain in the hills even this wide bed cannot carry the waters of the Son and disastrous floods in Shahabad, Gaya, and Patna are not uncommon."
The first obstruction in the Son was an anicut in 1873-74 at Dehri. The Indrapuri Barrage was constructed, 8 km upstream, and commissioned in 1968. The Bansagar Dam in Madhya Pradesh was commissioned in 2008.
The Government of Bihar sanctioned in 2008, a bridge across the Son River connecting Arwal and Sahar in Bojpur district. A four-lane road bridge, carrying NH 30, parallel to the existing rail and road Koilwar Bridge, has been planned.