The Kotmale River (Sinhala: කොත්මලේ ඔය) is the longest tributary of Mahaweli River. The river begins as the , in the Horton Plains. The Central Plains have an annual rainfall of between 125 and 200 inches a year. The river is approximately 70 km (43 mi) 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 , the , and the .