The dam creates the Randenigala Reservoir. With a catchment area of 2,330 km (1,450 mi) and a total storage capacity of 861,000,000 m3 (3.04×1010 cu ft), Randenigala is one of the largest reservoirs in the country.
The reservoir experiences approximately 1,250–3,000 mm (49.21–118.11 in) of rainfall annually. In addition to this, the reservoir is also topped up with water from the Victoria Reservoir upstream, and the Mahaweli River.