Thomson Dam

Thomson Dam
Thomson Dam Location of the dam in Victoria
Interactive map of Thomson Dam
Country	Australia
Location	West Gippsland, Victoria
Coordinates	37°50′34″S 146°23′56″E / 37.84278°S 146.39889°E / -37.84278; 146.39889
Purpose	Water supply Irrigation Power generation
Status	Operational
Construction began	1976
Opening date	May 1983 (1983-05)
Operator	Melbourne Water
Dam and spillways
Type of dam	Rock-fill dam
Impounds	Thomson River
Height (foundation)	166 m (545 ft)
Length	590 m (1,940 ft)
Dam volume	1.417×10^6 m3 (50.0×10^6 cu ft)
Spillways	1
Spillway type	Uncontrolled ogee-shaped overflow weir and chute
Spillway capacity	1,040 m3/s (37,000 cu ft/s)
Reservoir
Creates	Thomson Reservoir (official); known as Lake Thomson
Total capacity	1,123.09 GL (910,500 acre⋅ft)
Catchment area	487 km2 (188 sq mi)
Surface area	223 ha (550 acres)
Normal elevation	494 m (1,621 ft) AHD
Power Station
Type	Conventional
Installed capacity	7.4 MW (9,900 hp)
Annual generation	60 GWh (220 TJ)
Website melbournewater.com.au

The Thomson Dam is a major embankment dam across the Thomson River, located about 130 kilometres (81 mi) east of Melbourne in the West Gippsland region of the Australian state of Victoria. The reservoir (or storage basin) created by the dam is officially called Thomson Reservoir, sometimes called Lake Thomson.

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.^[1]

Early work in the early 1970s saw construction of a 19-kilometre-long (12 mi) 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 rock-filled dam wall is 166 metres (545 ft) high and 590 metres (1,940 ft) long. When full, the reservoir has capacity of 1,123 gigalitres (910,000 acre⋅ft) and covers 223 hectares (550 acres), drawn from a catchment area of 487 square kilometres (188 sq mi). The ogee-shaped overflow weir and chute spillway has a flow capacity of 1,040 cubic metres per second (37,000 cu ft/s). Also completed in 1983, an additional 36-metre-high (118 ft) and 580-metre-long (1,900 ft) rock-filled saddle dam is located off-stream, adjacent to the main dam wall, that added an additional 514 megalitres (417 acre⋅ft) of storage capacity to the reservoir.^[2]

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.

Downstream releases from Thomson Reservoir pass through a 7.4-megawatt (9,900 hp)^[3] conventional hydro power plant, at the base of the dam which generates electricity and feeds it into the state power grid.

Managed by Melbourne Water,^[4] between 1997 and 2011, drought depleted much of the reservoir's water.^[5][6] In early January 2006, the Thomson Dam was at 45.4%.^[7] While there were minor rises in water levels occasionally, the Thomson Dam reached its all-time low of 16.2% on 3 July 2009.^[8]

Heavy rainfall in 2010 and 2011 increased Melbourne's water storages to levels not seen for ten years.^[9] The Thomson Dam entered winter 2011 at 39% full^[10] and by the end of 2011 had reached 54.4% full.^[11]

On 28 October 2022, the dam reached 100% capacity and began spilling for the first time since the spring of 1996.^[12]

• List of reservoirs and dams in Australia § Victoria
• List of power stations in Victoria (state) § Hydroelectric

Media related to Thomson Dam at Wikimedia Commons