It has a drainage basin of 10,448 km2 (4,034 sq mi), the largest in the country, which covers almost one-fifth of the total area of the island.

[3] The Mahaweli Ganga starts at Polwathura (in the Mahawila area), a remote village of Nuwara-Eliya District in bank Nawalapitiya of Kandy District by further joining of Hatton Oya and Kotmale Oya.

The bay includes the first of a number of submarine canyons, making Trincomalee one of the finest natural deep-sea harbours in the world.

[5] The production of hydroelectricity from six dams in the Mahaweli system supplies more than 40% of Sri Lanka's electricity needs.

The Mahaweli gets its source waters from Horton Plains in Kirigalpoththa and the Thotupola mountain range.