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 Mahaweli Ganga river has a large drainage basin, covering almost one-fifth of the island's total area. Its origin is at Polwathura (Mahawila area) in Nuwara-Eliya District, where it joins Hatton Oya and Kotmale Oya to form its main course.