Located about 35 kilometres west of Sydney, Prospect Reservoir was completed in 1888 as part of the Upper Nepean Scheme to supply Sydney with water collected from the weirs on the Illawarra Plateau south of the city.
Between 1902-1935 four dams were added to expand the Upper Nepean Scheme 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.
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.
Height: 26 m
Length: 2.2 kilometres
Total operating capacity: 33,330ML
Lake: 5.2 km2
Catchment: 10 square kilometres
Why the reservoir was built
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.
As early as 1888, two weirs on the Cataract, and Nepean rivers captured this rainfall as part of the Upper Nepean Scheme to supply Sydney's growing water needs. A series of tunnels, canals and aqueducts - known as the Upper Canal - fed the water by gravity 64 kilometres to Prospect Reservoir. From there, water was fed by gravity along the 7.7 kilometre Lower Canal to a basin at Guildford and then water was piped to several reservoirs around the city.
The Upper Nepean Scheme provided only temporary relief, and the 1901 - 1902 drought brought Sydney perilously close to a complete water famine. After two Royal Commissions, authorities agreed that a dam should be built on the Cataract River. The successive building of Cataract, Cordeaux, Avon and Nepean dams between 1902 and 1935 greatly increased the Upper Nepean Scheme's capacity.
Capacity increased again in May 1940 when water from the Warragamba Emergency Scheme first reached Prospect Reservoir. The water was piped across Megarrity's Creek Bridge at Warragamba, and 26 kilometres to Prospect Reservoir. When Warragamba Dam was completed in 1960, Prospect Reservoir continued to play an important role in storing Sydney's water supply.
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.
When the Prospect water filtration plant was commissioned in 1996, raw water from Warragamba and the Upper Nepean dams was sent directly to the treatment facility, bypassing Prospect Reservoir.
However, the reservoir remains an integral part of Sydney's drinking water supply and is used regularly in times of high demand for water and when other parts of the water supply system are taken offline for maintenance. The filtration plant at Prospect can also draw water directly from Prospect Reservoir, providing greater flexibility in sourcing the best quality water.
How the dam was built
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 reservoir is located on Prospect Creek, with a catchment of only 10 square kilometres. However, the reservoir's main purpose was always as a storage reservoir for water collected elsewhere.
The reservoir is surrounded by Cumberland Plain woodland, one of the best examples of native bushland remaining in western Sydney. Wombats, echidnas and kangaroos are often seen in the bush surrounding the reservoir.
To meet modern dam safety standards, Prospect Reservoir's earthfill embankment was strengthened by placing stabilising berms on the downstream face in 1979, and the upstream face in 1997.