The Thomson Dam is an embankment dam across the Thomson River, located about east of Melbourne in the West Gippsland region of the Australian state of Victoria. Completed in 1983, the resultant eponymous reservoir is officially called Thomson Reservoir, sometimes called Lake Thomson and was created for the principal purpose of the supply of potable water for Greater Metropolitan Water.
The dam and reservoir are managed by Melbourne Water.
Dam and reservoir overview Dam The dam is located near the former township of Beardmore and the Baw Baw National Park. Despite opposition from conservationists and farmers, plans for the dam were originally approved in late December 1975 to provide Melbourne with water security. A dam on the Thomson River was preferred because the river had a large flow, high water quality and was elevated high enough to provide water to the upper Yarra system by gravity flow.
Early work in the early 1970s saw construction of a tunnel through the Thomson Yarra divide to allow water from the Thomson River to flow into the Upper Yarra Reservoir. Work on the dam itself commenced in 1976 and the completed dam and reservoir were ready to retain water by 1983. The dam was officially opened by Premier John Cain on 13 May 1983.
The rock-filled dam wall is high and long. When full, the reservoir has capacity of and covers , drawn from a catchment area of . The ogee-shaped overflow weir and chute spillway has a flow capacity of . Also completed in 1983, an additional and rock-filled saddle dam is located off-stream, adjacent to the main dam wall, that added an additional of storage capacity to the reservoir.
Reservoir The reservoir is Melbourne's largest water storage and accounts for approximately sixty per cent of overall capacity. Despite flooding the reservoir in 1983, it took another six years until the reservoir was full enough to provide limited water releases.
The tunnel, which is located at the northern end of the reservoir, allows water to be transferred west to Upper Yarra Reservoir and then on to Silvan Reservoir for distribution as drinking water in Melbourne.
Between 1997 and 2011, drought depleted much of the reservoir's water. In early January 2006, the Thomson Dam was at 45.4%. While there were minor rises in water levels occasionally, the Thomson Dam reached its all-time low of 16.2% on 3 July 2009. Heavy rainfall in 2010 and 2011 increased Melbourne's water storages to levels not seen for ten years. The Thomson Dam entered winter 2011 at 39% full and by the end of 2011 had reached 54.4% full. On 28 October 2022, the dam reached 100% capacity and began spilling for the first time since the spring of 1996.
Hydroelectric power station Downstream releases from Thomson Reservoir pass through a conventional hydroelectric power station, located at the base of the dam, which generates approximately of electricity annually that is fed into the state power grid. Commissioned in 1989, the plant was upgraded in 2012.
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