It was built by the Chola king Karikalan around the 1st Century AD and is considered one of the oldest water-diversion or water-regulator structure in the world, still in use. The Kaveri River forms the boundary between the Erode and Salem districts. The Bhavani River joins the Kaveri at the town of Bhavani, where the Sangameswarar Temple, an important pilgrimage spot in southern India, was built at the confluence of the two rivers. Sweeping past the historic rock of Tiruchirapalli, it breaks into two channels at the island of Srirangam, which enclose between them the delta of Thanjavur (Tanjore), the garden of South India.
The northern channel is called the Kollidam (Kolidam); the other preserves the name of Cauvery, and empties into the Bay of Bengal at Poompuhar, a few hundred miles south of Chennai (Madras). On the seaward face of its delta are the seaports of Nagapattinam and Karikal. Irrigation works have been constructed in the delta for over 2,000 years.
The Kallanai is a massive dam of unhewn stone, 329 metres (1,080 feet) long and 20 metres (60 feet) wide, across the main stream of the Cauvery. The purpose of the dam was to divert the waters of the Cauvery 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 Cauvery. The area irrigated by the ancient irrigation network of which the dam was the centrepiece was 69,000 acres (280 square kilometres). By the early 20th century the irrigated area had been increased to about 1,000,000 acres (4,000 square kilometres).