The Rivers of Sri Lanka originate in the central highlands. From there they descend to the plains and empty into the sea. The rivers are typically unnavigable in their higher reaches, where they flow swiftly and turbulently through highly eroded passages to the plains below. Many rivers descend over steep cliffs, forming spectacular waterfalls. In their lower courses, the rivers slowly meander through flood plains and deltas.

The longest river of Sri Lanka, the Mahaweli, traverses a course of about 330 km (about 205 mi). It flows northeastward across the central highlands and empties into the Bay of Bengal near the port of Trincomalee, on the eastern coast. The country’s second longest river is the Aravi Aru, traversing about 220 km (about 135 mi) on a northwestward course, from the central highlands to the Gulf of Mannār.

Sri Lanka has no natural lakes. Dams on the Mahaweli and other rivers have created large reservoirs. In addition, a series of small reservoirs called tanks dot the north central plains, storing water during the dry season. Some of the tanks were constructed as many as 2,000 years ago.

Much of Sri Lanka is arid and has only a few permanent rivers. However, the southwestern region’s “wet zone” is characterised by numerous rivers that arise in the high mountains of the central part of the island. These diverse river basins support endemic populations of aquatic plants, bivalves, and fish.

Sri Lanka’s known freshwater species include 90 fish (with twenty-six endemics) and 21 crabs, yet ongoing studies suggest that the number of undescribed species is potentially quite large.