The Ponnaiyar Basin is the second largest interstate East flowing river basin among the 12 basins lying between the Pennar and Cauvery basins. It covers a large area in the state of Tamil Nadu besides the areas covered in the states of Karnataka and Andhra Pradesh. It lies between East longitude 77° 33' to 79° 47' and North latitude 11° 45' to 13° 30' The Basin is bounded on the North -West and South by various ranges of the Eastern Ghats like the Velikonda Range, the Nagari hills, the Javadu hills, the Shevaroy hills, the Chitteri hills and the Kalrayan hills and in the East by the Bay of Bengal.

The Ponnaiyar drains an area of 16,019 Sq Kms out of which nearly 77 percent lies in Tamil Nadu. The State wise distribution of the drainage area is as follows:

Name of StateDrainage Area( Sq.Kms)Percentage of Total
Andhra Pradesh2101.3
Tamil Nadu12,27976.7

The Ponniyar or the Dakshina Pinakini river rises near Hongashenhalli village at an elevation of about 900 m above m.s.l at North latitude 13º 25' and East longitude 77º 58' in the Kolar district of Karnataka state. From its origin, the river Ponniyar generally flows in the Southern direction for a length of 79 km. through Kolar and Bangalore districts of Karnataka before entering the Dharmapuri district of Tamil Nadu. The river flows another 247 Kms generally in the South-Easterly direction in the districts of Dharmapuri, Vellore, Tiruvannamalai, Cuddalore and Villupuram . The river then flows in Easterly direction below the Tirukoyilur anicut for another 70 Kms before finding its way into Bay of Bengal. The river Ponnaiyar branches into two, the Gadilam and the Ponnaiyar below the Tirukoyilur anicut. The Gadilam joins the Bay of Bengal near Cuddalore and the Ponnaiyar near the Union Territory of Pondicherry. On its way, the river receives a number of small streams and rivulets.

The Ponnaiyar basin is covered by the Archaean rocks such as Pyroxene granulites, Quartzite, Ferruginous Quartzite, Amphibolites, Gneiss and Hornblende biotite gneiss with younger intrusive of Pegmatite and Dolerite in the central and western parts . The eastern part is covered by the cretaceous formation of argillaceous, calcareous sandstone with clay and limestone. The tertiary formation is of Cuddalore sandstones and the recent formation of river alluvium and coastal alluvium.

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