(Sinhalese: “Great Sandy River”), river, central and eastern Sri Lanka. At 208 mi (335 km) in length, it is Sri Lanka's longest river. It rises on the Hatton Plateau on the western side of the island's hill country, flows north through a tea- and rubber-growing region, and turns east near Kandy; it then turns north across the lowlands, receives its principal tributary, the Amban Ganga, and flows past Polonnaruwa to its mouth on Koddiyar Bay, 7 mi south of Trincomalee.

With its headwaters in Sri Lanka's wet zone, the Mahaweli Ganga flows throughout the year, providing water for agriculture in the eastern dry zone. In the early 1970s a vast development project to increase the river's usefulness for irrigation and generation of electricity was under way, under the auspices of the World Bank. It is scheduled for completion by about 1990.