Last reviewed 1 Oct 2026 · Facts as of 1 Oct 2026 · 7 min read
For the TNPSC AE syllabus. Expect two kinds of questions: facts (which dam is on which river, year, features) and planning concepts (basin-wise planning, tank irrigation, groundwater and conjunctive use). Figures such as storage capacities are quoted approximately; use your prescribed text for exact numbers.
1. Hydrologic setting
| Item | Detail |
|---|---|
| Rainfall pattern | Tamil Nadu gets most of its rain from the north-east (retreating) monsoon, October–December; the south-west monsoon (June–September) is weak because of the rain-shadow of the Western Ghats |
| Average annual rainfall | Roughly 900–1,000 mm; large spatial and annual variation |
| Rivers | Mostly rain-fed, non-perennial rivers, except the Cauvery (partly fed by the south-west monsoon in Karnataka and Kerala) |
| River basins | The state is divided into 17 major river basins (plus the sub-basins) |
| Water availability | Per-capita water availability is low compared with the national average; the state depends on inter-state rivers (Cauvery, Palar, Periyar and others) |
| Water-surplus areas | Delta region (Cauvery) and parts of the southern basins |
| Water-deficit areas | Interior districts and the Chennai metropolitan area |
2. Major rivers and basins
| River / basin | Origin | Notes |
|---|---|---|
| Cauvery (Kaveri) | Talakaveri, Brahmagiri hills, Karnataka | Enters Tamil Nadu at Hogenakkal; the Mettur Dam and the delta (Thanjavur, Tiruvarur, Nagapattinam); tributaries include Bhavani, Noyyal, Amaravathi; Kollidam is a distributary |
| Palar | Nandi hills, Karnataka | Flows through Vellore, Kancheepuram; shared with Karnataka and Andhra Pradesh |
| Pennaiyar (Thenpennai) | Karnataka | Sathanur Dam |
| Vaigai | Western Ghats (Varushanadu hills) | Vaigai Dam (Theni district); supports Madurai; the dam was built in the late 1950s |
| Tamirabarani (Thamirabarani) | Western Ghats (Agasthyamalai) | Perennial river of the south; flows through Tirunelveli and Thoothukudi |
| Vellar | Shevaroy hills | Cuddalore region |
| Bhavani | Nilgiris (Silent Valley region, Kerala) | Bhavanisagar Dam (Lower Bhavani Project) |
| Periyar | Western Ghats | Flows west into Kerala; its water is diverted to the Vaigai basin via the Mullaperiyar Dam |
| Kosasthalaiyar (Kortalaiyar), Cooum, Adyar | — | Chennai rivers; seasonal and polluted in the city |
3. Historic irrigation structures
- Kallanai (Grand Anicut): on the Cauvery, built by the Chola king Karikala Chola (about 2nd century CE); one of the oldest functioning water-regulation structures in the world; it diverts water into the delta. The British engineer Arthur Cotton improved it in the 1830s.
- Lower Anicut (Kollidam) — built by Arthur Cotton on the Kollidam.
- Mettur Dam (Stanley Reservoir): on the Cauvery in Salem district; completed in 1934 (named after the Governor Sir George Stanley); one of the largest masonry dams in the world at that time; opening of delta irrigation to the Cauvery Mettur Project.
- Veeranam Tank (Cuddalore) — an ancient large tank attributed to the Cholas; a source for Chennai through the Veeranam pipeline.