The Deduru Oya is the fourth largest river in the country flowing through Kandy , Matale, Kurunegala and Puttalama Districts. The river originates from the Western slopes of the Kandy Matale Hills. Catchments area of the river is 2620 sq.km. Annual discharge of the river is more than 1000 MCM. About 90% of the Deduru Oya catchment area comes under intermediate zone and rest of the area in mountainous region falls under mid country wet zone.
The annual precipitation in the basin varies from 1100 mm in the coastal region to 2600 mm in mountainous region. Deduru Oya basin receives about 50% of the annual rainfall during inter monsoon months (March, April, October and November), about 35% during South-West monsoon months (May to September), while remaining 15% during North-West monsoon months (December to February).
The major tributaries of the Deduru Oya namely Ratwila Ela, Dik Oya and Kospothu Oya in the upper reaches of the basin and Kimbulwana Oya, Hakwatuna Oya and Maguru Oya in the middle reaches of the basin confluence with the river upstream of the proposed dam site, while the other tributaries namely Kolamunu Oya , Thalagalla Ella meet the river downstream of the proposed dam site.
Deduru Oya and Mee Oya are two adjacent river basins located in the North Western Province. More than 80 percent of the water resource in the water deficit Mee oya basin is utilized for cultivation. But in the Deduru Oya basin much of the water resource has not yet been utilized. Lands in the Deduru Oya basin are almost developed and lands in the Mee Oya basin yet to be developed. Therefore it is planned to exploit the Deduru Oya water resources in improving the cropping intensity of existing agricultural lands under minor irrigation schemes and to develop new lands in the Mee Oya and the Deduru Oya basins in view of enhancing productivity.