The O'Shannassy Reservoir is a small reservoir in the south of the Australian state of Victoria in the course of the O'Shannassy River , above the small town of McMahons Creek , about 80 km east of Melbourne . The reservoir is part of Melbourne's drinking water supply. The water flows under dynamic pressure to the Silvan Reservoir and then on to the storage and distribution basins in the Melbourne area. The O'Shannassy Reservoir is the smallest reservoir operated by Melbourne Water with an area of 27 hectares . Its storage space is only about 3.1 million m³, but the catchment area is very water-rich, which leads to an average annual flow of about 80 million m³ / year.
The location was chosen because it is high enough to supply the higher eastern suburbs of Melbourne only with the help of hydrostatic pressure. The bypass weir in the O'Shannassy River and the aqueduct to the Surrey Hills Reservoir in Melbourne were completed in 1914. The diversion weir was replaced by the reservoir in 1928. The construction of the Yarra-Silvan lines in the 1950s reduced the need for the aqueduct and in 1997 it was decommissioned.
