The catchment of the upper Taf Fawr was but it was split into an upper area of and a lower one of . Any works had to provide compensation water to maintain the flow in the Taf Fawr, and working on the upper area first meant that the Corporation only had to supply per day of compensation flow until work started on the lower area. Cantref Reservoir, the lower of the two in the upper area, was constructed first. Work began on 4 May 1886 and was completed on 14 September 1892. Construction of Beacons Reservoir, the highest in the chain, began shortly afterwards, in April 1893, in advance of the Corporation obtaining a second Act of Parliament to increase the capacity of the reservoir and to move the dam further upstream, where the foundations would be better, in the light of geological investigation. Impounding of the water began on 17 September 1897.