O'Shannassy Reservoir
| O'Shannassy Dam | |
|---|---|
 Location of the O'Shannassy Reservoir in Victoria
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| Country | Australia |
| Location | McMahons Creek, Victoria |
| Coordinates | 37°40′30″S 145°48′20″E / 37.67500°S 145.80556°ECoordinates: 37°40′30″S 145°48′20″E / 37.67500°S 145.80556°E |
| Purpose | Water supply |
| Status | Operational |
| Construction began | 1922 |
| Opening date | 1928 |
| Operator(s) | Melbourne Water |
| Dam and spillways | |
| Type of dam | Embankment dam |
| Impounds | O'Shannassy River |
| Height | 34 m (112 ft) |
| Length | 226 m (741 ft) |
| Dam volume | 245×103 m3 (8.7×106 cu ft) |
| Spillways | 1 |
| Spillway type | Uncontrolled |
| Spillway capacity | 500 cubic metres per second (18,000 cu ft/s) |
| Reservoir | |
| Creates | O'Shannassy Reservoir |
| Total capacity | 3.123 GL (687,000,000 imp gal; 825,000,000 US gal) |
| Catchment area | 11.9 ha (29 acres) |
| Surface area | 27 ha (67 acres) |
The O'Shannassy Reservoir is an Australian man-made water supply dammed reservoir. The 3.1-gigalitre (680,000,000 imp gal; 820,000,000 US gal) water store across the O'Shannassy River is located near the locality of McMahons Creek, approximately 80 kilometres (50 mi) east of Melbourne, Victoria. The dam that creates the impoundment is called the O'Shannassy Dam.
The reservoir is part of Melbourne's water supply system. Water flows under gravity to Silvan Reservoir, then to storage and distribution reservoirs around Melbourne. It is the smallest of the water storage reservoirs managed by Melbourne Water, with a capacity of approximately 3.1 gigalitres (680,000,000 imp gal; 820,000,000 US gal), but it is on a very productive catchment, with stream flow averaging 80 gigalitres (1.8×1010 imp gal; 2.1×1010 US gal) per annum.
The location was selected as it is at sufficient altitude for gravity supply to the elevated eastern suburbs of Melbourne. A diversion weir on the O'Shannassy River and aqueduct to the Surrey Hills Reservoir in Melbourne were completed in 1914. The weir was replaced by the reservoir in 1928. The construction of the Yarra-Silvan conduits in the 1950s resulted in reduced requirements for the aqueduct and it was decommissioned in 1997.
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