Sri Lankan Rivers

Friday, November 27th, 2020 | Blog

River outflows provide the raw organic material at the base of the marine food chain. A total of 103 perennial and seasonal rivers flow from Sri Lanka into the surrounding seas, which are also fed by the numerous rivers and streams draining the southern Deccan Peninsula in India. There is a high degree of variation, both seasonal and geographical, in these outflows, and this in turn has a strong influence on marine resources and thus on marine mammal behaviour and sightings.

River outflows from Sri Lanka are greatest around the south-western quarter of the island, the so-called Wet Zone. Wet Zone rivers, nearly all perennial, account for 38 percent of the total surface water discharge from the island. Most Dry Zone rivers®, by contrast, are seasonal, only reaching the sea during the monsoon months, when rainfall is high. Of the relatively few perennial Dry Zone rivers, the greatest is the mighty Mahaweli Ganga, Sri Lanka’s longest river, which drains much of the central plain of the island as well as the eastern hill country, and accounts for about fifteen percent of the total surface water discharge from the island. Thanks in large measure to this single river, whose wanderings down the ages have helped form the complex geography and oceanography of Trincomalee with its multitude of bays and inlets and its vast offshore submarine canyons, the eastern seaboard accounts for 27 percent of all river outflows from Sri Lanka.

The rivers of the Deccan also play their part in nourishing the waters round Sri Lanka and immediately beyond. The 1,600km-long range of coastal hills known as the Western Ghats supports a 45-river system that flows into the seas round Kerala, Tamil Nadu and Andhra Pradesh. This region experiences heavy rain during the southwest monsoon and is a very important source of river outflows; indeed, it is the main watershed of the Deccan Peninsula, a region that accounts for about 40 percent of India’s drainage. During both monsoon periods, the Gulf of Mannar is fed primarily by the Thamirabarani River in Tamil Nadu, while Palk Strait is fed mainly by the Vaigai River in Kerala. Outflows from rivers and streams in the southernmost reaches of the Ghats, which lie in the state of Kerala and Tamil Nadu, are vital to the ecology of the south-eastern Arabian Sea, a critical marine mammal habitat.

Text by Howard Martenstyn, Out of the Blue