Prospect Reservoir was the centrepiece of the Upper Nepean scheme to secure Sydney’s water supply, devised by Edward Orpen Moriarty, engineer-in-chief of the harbours and rivers branch of the Public Works Department.
The Upper Nepean catchment south of Sydney is in one of the highest rainfall zones on the mid-NSW coast, and the area’s rivers, located in narrow gorges, provided ideal dam sites.
Located about 35 kilometres west of Sydney, work on Prospect Reservoir began in 1880 and was completed in 1888. It was the first earthfill embankment dam in Australia. The embankment is 26 metres high and 2.2 kilometres long. It consists of a clay core with shoulders of selected earth placed in layers 30 centimetres thick, and compacted by rolling. The top of the embankment was raised by half a metre in 1898. The embankment was further strengthened by placing stabilising berms on the downstream face in 1979, and the upstream face in 1997.
The reservoir is located on Prospect Creek, with a catchment of only 10 km2. However, the reservoir’s main purpose was always as a storage reservoir for water collected elsewhere, initially from weirs at the Illawarra Plateau south of the city. The reservoir is surrounded by Cumberland Plain woodland.
Between 1902-1935 four dams were added to expand the Upper Nepean Scheme (Cataract, Cordeaux, Avon and Nepean) and boost supplies to Prospect Reservoir. Even after Warragamba Dam was completed in 1960, Prospect Reservoir continued to play an important role in storing Sydney’s water. A second pipeline, linking Warragamba Dam and Prospect Reservoir was completed in 1966, significantly increasing the volume of water that could be transferred during peak demand periods.
Today, water from Warragamba and the Upper Nepean dams bypasses Prospect Reservoir. However, the reservoir remains an integral part of Sydney’s drinking water supply and is still used regularly in times of high demand for water and when other parts of the water supply system are taken offline for maintenance.
We visited Prospect Reservoir on 11 April 2021 in the afternoon. There is a historic Valve House used for outflows from the reservoir but today the water is channelled into the Prospect water filtration plant when required. The picturesque Outlet Tower is designed in a Gothic Revival style and there are a lot of picnic grounds nearby.