It also serves as a model for the engineers across the world and continues to play a major role in water distribution to the Cauvery Delta region, the rice bowl of Tamil Nadu.

Background: The Kallanai Dam was constructed during the second century by king Karikalan of the Chola Dynasty, and is considered to be one of the oldest irrigation systems in the world that is still in use.

With the construction of the dam, Karikalan changed the fate of the Chola Kingdom forever. He did not just think of the near future, he thought about a structure that can stand for a very long time. 

The Cauvery delta region has always been fertile. The river caused several floods during the monsoon seasons and the delta experienced droughts during the dry months. Hence, to change these extremities, Karikalan and his advisers devised the project to maintain a steady flow of water throughout the year. 

Originally, the river split into two (Northern channel was Kollidam and the southern channel retained the name Cauvery) at Srirangam and they came close again downstream. Karikalan built the Kallanai on the southern channel where it came close to Kollidam.

Structural details: Kallanai splits Cauvery into four major branches - Kollidam, Cauvery, Vennaru and Puthu Aru - and they played major role in irrigation and thereby improving the cultivation of food grains. Thanjavur, which once had to import food grains, soon became the rice bowl of south India with its construction.

Besides diverting excess flood water into four smaller streams, he also built canals to utilise the water for irrigation. 

At the time of construction, the structure was 329 m long with uneven stones, 20 m wide and 5.4 m high. It was built with large boulders sunk in the Cauvery sand, a task arising of a desperate need for irrigating fertile fields downstream when the floods breached the left bank and rushed down north to re-join the Kollidam (river Coleroon), leaving its delta region dry. 

The area irrigated by the irrigation network, of which the dam was the centrepiece, was 69,000 acres. Around 20th century, it was increased to about 10,00,000 acres.

When the flow of water to the delta districts was stopped in due course of time, the Kallanai appeared to be a heap of sand and the district witnessed drought.

The British venerate: In 1829, the then British government appointed Sir Arthur T Cotton, the father of Indian irrigation, to supervise the irrigation works in the Cauvery region. 

Cotton, who renamed the dam as ‘Grand Anaicut’, described it as ‘wonders of engineering by Tamils’ in his book. Later, he studied the technicalities in detail and constructed dams and bridges across the region employing the same techniques.

He said the dam, which was built by Karikalan 18 centuries earlier, will withstand for years for its strength and it will not also need renovation in future.