Mundaring Weir, Western Australia
| Lake C.Y. O'Connor Helena River Reservoir |
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| Location | Mundaring, Western Australia |
| Coordinates | 31°59′S 116°12′E / 31.983°S 116.200°E |
| Type | reservoir |
| Primary inflows | Helena River |
| Primary outflows | Helena River |
| Basin countries | Australia |
| Surface area | 6.76 km2 (2.61 sq mi) |
Mundaring Weir is a dam (and historically the adjoining locality) located 39 kilometres (24 mi) from Perth, Western Australia in the Darling Scarp. The dam and reservoir form the boundary between the suburbs of Reservoir and Sawyers Valley. The dam impounds the Helena River.
A soldier, Ensign Robert Dale, became the first European to explore the region in 1829.
European populations did not grow significantly until construction of the dam in the late 1890s. This involved the building of a railway line from Mundaring to the Mundaring Weir site. The Irish Australian engineer C. Y. O'Connor was involved in the design of a scheme that transported water to the Eastern Goldfields of Coolgardie and Kalgoorlie in the eastern part of Western Australia.
The weir was completed in 1903.
The lake created by the dam was known as the Helena River Reservoir, however it is now known as Lake C.Y. O'Connor.
The owner of the dam, the Water Corporation, refers to the weir as Mundaring Dam on its website, but no other authority, such as Geographic Names, or Geosciences Australia uses this term. The Mundaring Shire uses an image of the Mundaring Weir in its logo.
Work commenced to raise the dam in the late 1940s, and was completed in November 1951.
In the early 1970s the downstream dam from the weir—the Lower Helena Pumpback Dam—was constructed.
It last overflowed into the Helena Valley in 1996.
Like any structure of its age and design, the weir requires monitoring and repairs and maintenance. On the original structure the central small building was used for aspects of monitoring the weir's internal status – it was rebuilt on the heightening of the dam, although it was no longer used for its original purpose.
In 2009 two face wall entrance points were created on either side (north and south) of the face to facilitate access to the monitoring tunnels. The weir leaks, and the two photos following show consistent stain/moisture points between 2008 and 2010 where water moves through the structure:
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