Science News

Submerged river systems hint at India-Sri Lanka water links

K. S. Jayaraman

doi:10.1038/nindia.2019.91 Published online 18 July 2019

Its capital Chennai may now be facing a water shortage, but once upon a time the south Indian state of Tamil Nadu was a generous exporter of its river waters, according to a study1 based on satellite data. Vaigai was once a major river that flowed from Tamil Nadu all the way up to Galle in the island nation of Sri Lanka, it says.

The river was submerged due to sea level rise, says the report by Professor of Eminence Somasundaram Ramasamy and colleague J. Saravanavel, at the Department of Remote Sensing at Bharathidasan University in Tiruchirappalli. They also report that Tamiraparani, another river in Tamil Nadu, was a tributary of Vaigai in the past. The waters of these two South Indian rivers had a Sri Lankan link, their report says.

The authors say these two rivers might have been be more than 20,000 years old. The land over which they flowed was swallowed by the sea and is now several metres under water in today’s Gulf of Mannar (GoM) — the region separating India and Sri Lanka.

Their curiosity, prompted by the “abrupt truncation” of the delta fronts of both Vaigai and Tamiraparani, led them to search for any trace of these rivers offshore on the sea floor in the GoM region.

© Ramasamy, S. M. & Saravanavel, J.

Using bathymetric (underwater topography) data downloaded from a public website (GEBCO) and geographic information system software (ArcGIS), the researchers created images of the ocean floor which showed “well-defined, depressions and gorges” resembling river valleys in the offshore regions of Vaigai and Tamiraparani rivers.

Further analysis using data sent by an Indian Remote Sensing satellite revealed that one of the well defined depressions coincided with the Vaigai river mouth. From there it extended eastward in the ocean to north of Rameswaram Island, and then in a southerly direction for more than 400 km up to west of Galle in Sri Lanka. “Such morphology of the depression indicates that it is a river-cut valley and may be the old path of Vaigai river in the GoM region,” the researchers say.  

Similarly, another major depression in the ocean floor coincided with the mouth of Tamiraparani river. The researchers noted it might have been the path along which the Tamiraparani flowed in the past and in that process carved out the valley.

Having found these submerged valleys, the researchers concluded that "Vaigai might have been the major river that flowed in the past in GoM region and then taken a southerly turn towards Sri Lanka, and the Tamiraparani river might have been a tributary of Vaigai.”

Analysis of past sea-level data indicated that the sea level has been continuously rising since the last 20,000 years. When this sea-level data were matched with corresponding shorelines that existed during those periods, “it indicated that the two rivers have gradually submerged due to rise in sea level that started 20,000 years ago."

The discovery of submerged river valleys in the GoM region has revived the debate over ‘Kumari Kandam’, a mythical continent, with an ancient Tamil civilization that, according to some scholars, existed south of present-day India in the Indian Ocean.

“We located the buried Vaigai in the Gulf of Mannar region. This has raised a lot of curiosity about the lost Kumari Kandam,” Ramasamy says.On whether there was any evidence yet to support the existence of the disputed mythical continent, he says, "Yes, but not that huge — we have located it to be around 2-3 lakh square kilometres.”

However, Chittenipattu P. Rajendran, a geophysicist at the Jawaharlal Nehru Centre for Advanced Scientific Research in Bengaluru is skeptical. “The authors have talked about the changing sea level over the last 20,000 years and the extension of the Vaigai river flood plain further on to the continental shelf,” he says, “But I don't see any of this bearing on the lost continent”.


References

1. Ramasamy, S. M. & Saravanavel, J. Drowned valleys of Vaigai and Tamiraparani rivers in the Gulf of Mannar region, India. Curr. Sci. (2019) Article