It has a drainage basin of , the largest in the country, which covers almost one-fifth of the total area of the island. The real beginning of Mahaweli Ganga starts at Polwathura (at 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 drainage basin of , the largest in the country, which covers almost one-fifth of the total area of the island. The real beginning of Mahaweli Ganga starts at Polwathura (at Mahawila area), a remote village of Nuwara-Eliya District in bank Nawalapitiya of Kandy District by further joining of Hatton Oya and Kotmale Oya.