The
Kotmale River is the longest tributary of
Mahaweli River. The river begins as the Agra Oya, in the
Horton Plains. The Central Plains have an annual rainfall of between 125 and 200 inches a year. The river is approximately long and drains a basin of about 58,534 ha (226 square miles). The river flows through a traditional area of ancient villages and tea plantations. King Dutugemunu spent his youth here. The Kotmale Oya flows into the Mahaweli shortly at
Pallegama. Tributaries of the Kotmale Oya include the
Nanu Oya, the Pundal Oya, the Puna Oya, and the Dambagastalawa Oya.