It starts at an elevation of about 1,800 m on the southern wall of the Adam’s Peak range (not from the Peak) to the southern coast traversing a distance of 138 km and discharging an annual average volume of 2,200 Mm3. It is the fourth largest river in the country in terms of volume discharged and has a catchment of 2,456 km2 served mostly by the two inter-monsoonal rains peaking in April and November. The annual average river flow at site is 18 m3/sec and the minimum flow recorded is 3.5 m3/sec. Samanalawewa is the second reservoir built across Walawe, the first being the Uda Walawe Reservoir built in 1965 for the purpose of “developing” the southern plains which was at that time covered with virgin jungle. With the availability of irrigated water, the region is now converted into paddy fields and cash-crop plantations such as sugar cane and banana with corresponding growth of new settlements.
The southern plains of the country rises to an elevation of about 100 m gradually and then within a matter of a kilometre rises to over 400 m elevation forming the southern wall. The upper region terrain is steeply hilly and undulated, and Walawe and its main tributary Belihul Oya flow along deep valleys in it. The two waterways, after their confluence, cascade down to the lower plains within a short distance. Walawe is damned just below the confluence forming the Samanalawewa. Flowing along the lower elevation is Katupath Oya which joins Walawe after merging with another tributary Diyawini Oya. At one location, Katupath Oya flows only a short distance away from Walawe but more than 300 m below. Water from the reservoir is taken down this height difference into Katupath Oya through a power plant driving a generator with installed capacity of 120 MW. The power plant was initially estimated to generate 460 GWh of electricity annually, which was about 15% of the total hydro energy output in 1990.
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