Nepean Dam was the last and smallest of the four dams constructed as part of the Upper Nepean Scheme. Unlike the earlier dams with Tudor and Egyptian styles, this dam captured the spirit of the Roaring Twenties and the Swinging Thirties through it’s Art Deco style.
Construction started in 1925 but stopped for two years during the Great Depression. The dam was not completed until ten years later in 1935. The dam wall is relatively high at over 80m, with a length of over 200m. The catchment area is only 320 km2 and the lake size is 3 km2 for a total capacity of just over 65 gigalitres.
Nepean Dam’s main role today is to supply water to the nearby towns of Bargo, Thirlmere, Picton and The Oaks, as well as the Macarthur and Prospect water filtration plants, but it also receives water transferred from the Shoalhaven Scheme (Tallowa, Fitzroy Falls and Wingecarribee). Like the other dams in the Illawarra Plateau, it forms part of Sydney’s water supply network.
Similar to the other Upper Nepean dams, the wall was built using cyclopean masonry – sandstone blocks, quarried from the site, fitted into an irregular pattern and packed with sandstone concrete. Some of the plant and equipment used in the construction of Avon Dam were transferred to Nepean Dam.
A tunnel linking Avon and Nepean Dams was built in 1973, and in 2006 a pumping station enables forced water transfer in either direction regardless of dam levels.
When we visited in late afternoon on 11 April 2021, the spillway was in operation, creating a dramatic cascade of water from the lake culminating in spectacular falls downstream (which, sadly, we were not allowed to get close to). We also saw terraced structures and the remnants of what would have been at one stage extensive landscaped gardens for people to enjoy when having a picnic. There is a row of quaint picnic shelters leading across a path flanked by Roman Cypress trees that eventually end in a lookout of the lake.