A river's diet
The Periyar River, at 244 kilometers long, is Kerala’s largest river and has historically been the primary architect of its central coastline. For millennia, it carried sand, silt, and clay from the Western Ghats mountains, depositing this material at its mouth near Kochi. This constant supply of sediment built and maintained the coastline, forming barrier islands like Vypin and nourishing the vast Vembanad-Kol wetland system. The river's powerful freshwater discharge also acted as a natural barrier, pushing back the salt water of the Arabian Sea and maintaining an estuarine ecosystem. The balance defined the region's geography and supported its fisheries.
Engineering a new coastline
The 20th century saw the construction of numerous dams along the Periyar and its tributaries. The process began with the Mullaperiyar Dam, completed in 1895, but accelerated significantly with projects for hydroelectricity and irrigation. Important structures include the massive Idukki Dam, a 169-meter-high arch dam finished in 1973, which is one of the highest in Asia. Today, at least 17 dams regulate the river's flow.
These dams, while providing power and water, are effective sediment traps. Studies estimate that the total sediment trapped in the Periyar's reservoirs is about 30 times the amount the river transported to the coast between 1978 and 2002. This has placed the coast on a sediment starvation diet, reducing the delivery of sand and silt by as much as 80%. With the river no longer replenishing the shore, the erosive power of waves has become dominant.
Erosion is visible along the coast. About 63% of Kerala's 590-kilometer coastline now experiences erosion. Islands at the river's mouth show changes. On Vypin Island, peak erosion rates reached over 64 meters during a four-year period from 2015 to 2019. The reduced freshwater flow also allows saltwater to penetrate much farther upriver. In the dry, pre-monsoon season, saltwater intrusion can now be detected more than 21 kilometers upstream, threatening drinking water intakes for the city of Kochi and disrupting the brackish water conditions essential for local prawn and clam fisheries.