by constructing a concrete channel approximately long, wide, widening to at the discharge weir. Construction of the channel commenced in July 1943, and was completed in November 1947. In the late 1960s, as part of a plan to interconnect the various water supply systems for Sydney Wollongong and Sutherland, a tunnel was constructed between the Nepean Dam and the Avon Dam (which now supplies the Wollongong area). This allows water to be transferred in either direction, as required. In the 1980s, work was undertaken on Nepean Dam to alleviate uplift pressure on, and leakage through, the foundations of the dam wall, in accordance with modern design criteria. The downstream face of the wall was supported with an embankment of quarried sandstone blocks and sandstone fill, the latter being compacted by grid and vibratory rollers. It was similar to the work undertaken at the same time for Avon Dam. Description The Nepean Dam is a large cyclopean masonry dam. This consists of large blocks of sandstone quarried at the site, embedded in and surrounded by sandstone concrete. To ensure water-tightness, all construction joints are grouted. The upstream face consists of blue metal concrete thick while the downstream face is of similar material thick. Both of the facing concrete sections are merged progressively into the central sandstone masonry concrete. All the latest improvements in dam design and construction of the day were embodied in the work, building upon the experience gained from the construction of the previous three Upper Nepean Scheme dams. Inspection galleries are provided, with pressure and drainage pipes from the foundations, and rubble drains lead into them. Provision is made for any leakage of water past the copper seals in the contraction joints to be conducted into the galleries and thence drained away. To facilitate the work of maintenance staff, an electric lift was provided in the wall. The water is discharged through two diameter outlet pipes controlled by Ring Falkner gate valves and FCD valves. After construction was complete and the dam had filled, the spillway weir was altered by the construction on the western side of the stream of a concrete channel approximately long and wide, widening to at the discharge weir. The Nepean Reservoir has a small storage capacity in relation to its large catchment area. While its capacity is much the same as that of Cataract and Cordeaux Reservoirs, its catchment basin is two and