Vellar River (Southern Tamil Nadu)

The river's drainage basin makes up the southern and westernmost part of the Agniyar basin, one of Tamil Nadu's 17 major river basins, which is bounded to the north by the Kaveri basin and to the south by the Pambar basin. Two other rivers’ sub-basins, those of the Ambuliyar and Agniyar, border to the northeast and northwest of Vellar's watershed respectively, and make up the rest of the Agniyar basin. The river's sub-basin covers 2034 km² of land mostly in Dindigul, Tiruchirappalli, and Pudukkottai districts, although smaller areas in Sivaganga and Thanjavur districts are also within the watershed. In 2007, the sub-basin was inhabited by about 805,000 people.^[1]

The Vellar River is extensively dammed in order to draw from the river, mostly for agricultural purposes such as irrigation, although it is also used to supply water for industrial and domestic demand. Along its course, the river is dammed 17 times, and water from it is diverted to a total of 161 storage tanks/retention basins with an approximate total capacity of 270 million cubic meters, and to 13,213 acres of farmland.^[1]

Total demand consistently exceeds total potential, resulting in annual water deficits which prevent the river from actually discharging into the sea. Surplus flow in the Vellar was last recorded in 1993, with a registered volume of 446 million cubic meters. Compared to the nearby Agniyar and Ambuliyar rivers, which do consistently reach the sea during each year's wet season, the Vellar lacks significant tributaries towards its mouth such as the Maharajasamundram, the main tributary of the Agniyar River, which joins the Agniyar only a short distance from where it discharges. In comparison, the Nerunjikudiyar and Gundar rivers are much smaller and further upstream.^{[citation needed]}