The river has a total length of 150 kilometres (93 mi), and its drainage basin covers an area of 7,504.346 square kilometres (2,897.444 sq mi) in Cuddalore, Perambalur, Salem, Kallakurichi, Ariyalur, Namakkal, Tiruchirappalli, and Dharmapuri districts.

[2] The Vellar continues southeast, passing south of Ethapur, northeast of Pethanaickenpalayam, and southwest of Kalpagnur.

South of Velluvadi, the Ellar River joins the Vellar on its right bank and from the west.

[6] East of V.Kalathur, the Vellar is again joined on its right bank and from its west by the Swetha River, its second largest tributary.

It flows between Killai and Parangipettai, and into the Bay of Bengal, in Chidambaram taluk, Cuddalore district, where it forms a small bar-built estuary.

The basin is usually divided into seven sub-basins, which correspond to the drainage area of a major tributary within the river system.

There are 5 major bodies of water, those being the Anamaiduvu, Kariakovil, Gomukhi, and Manimuktha reservoirs, along with Wellington Lake.

Generally, the basin is more flat, cultivated, and level in the east, while it tends to be more hilly and forested to the west and inland.

Structural hill and valley fill denudational landforms are most common in the west.

In the central region and towards the mouth of the river, fluvial landforms and floodplains are more common.

[2] The Upper and Lower Vellar irrigate a total area of 424.62 square kilometres (163.95 sq mi) of land for agricultural purposes.

The greatly reduced flow during the dry season generally cannot handle water demands from various sources.