The proposed study area is located within the Balui sub-watershed of the upper Rajang River Basin in the interior of Sarawak. The Bakun catchment area is located between latitudes 1.5°N and 3.0°N and longitudes 113.5°E and 115.3°E. The catchment upstream of the dam site covers an area of about 1.5 million hectares (ha). The watershed and river are respectively the largest (44,200 km2) and the longest (>900 km) in Malaysia and the Balui or Upper Rajang sub-watershed represents 34% of the entire Rajang watershed.