The Grand Anicut, also known as the Kallanai (Tamil: கல்லணை) (kall -stone, anai-bund), is an ancient dam built on the Kaveri River in the state of Tamil Nadu in southern India.It was built by the Chola king Karikalan around the 2nd Century AD. and is considered one of the oldest water-diversion or water-regulator structures in the world, which is still in use.
The Kallanai is a massive dam of unhewn stone, 329 m (1,079 ft) long and 20 m (66 ft) wide, across the main stream of the Kaveri. The purpose of the dam was to divert the waters of the Kaveri across the fertile delta region for irrigation via canals. The dam is still in excellent repair, and supplied a model to later engineers, including the Sir Arthur Cotton's 19th-century dam across the Kollidam, the major tributary of the Kaveri. The area irrigated by the ancient irrigation network is about 69,000 acres (28,000 ha). By the early 20th century, the irrigated area had been increased to about one million acres.