Thomson is Melbourne’s largest reservoir, making up about 60% of Melbourne’s total water storage capacity. It was designed to build up our water reserves in wet years, for use when conditions are dry.
Thomson Reservoir is filled by rainfall received by the surrounding catchment. It can transfer over one billion litres of water per day to Upper Yarra Reservoir, which in turn supplies Silvan Reservoir. From Silvan, water can be distributed throughout metropolitan Melbourne.
The dam at Thomson Reservoir is an earth and rockfill embankment, completed in 1984.
- Catchment area: 48,700 hectares
- Surface area: 2,230 hectares
- Total capacity: 1,068,000 megalitres
- Dam height: 165 metres
- Dam length: 590 metres (main bank only, saddle dam is an extra 580 metres)
Thomson is the third-largest reservoir to supply a major Australian city, behind Warrangamba Dam (New South Wales) and Wivenhoe Dam (Queensland). It is large enough to contain all of the dams servicing Canberra, Adelaide and Perth!
Put another way, it can store almost four times as much as Melbourne’s next largest reservoir, Cardinia Reservoir, or enough water to fill:
- 2 Sydney Harbours
- 628 MCGs
- 427,200 Olympic swimming pools.
As well as servicing Melbourne in times of drought, Thomson Reservoir also provides water to maintain healthy flows in the Thomson River, support agriculture in the Macalister Irrigation District downstream, and generate hydroelectricity.
Impact on storage capacity
Thomson Reservoir was designed to serve a city of 5 million people, based on rainfall and water use at the time it was proposed in 1968.
On 31 July 1984, the newly-built and virtually empty reservoir was officially factored into Melbourne’s water storages. This more than doubled Melbourne’s total storage capacity, and dragged down storage levels from 65% to 26% overnight.
It has been full three times: 1992, 1993 and 1996.
Rainfall and storage levels over the years
During the 1997-2009 Millennium Drought, record low inflows severely tested the commonly-held notion that Thomson would ‘drought proof’ Melbourne. Its storage levels dropped from full to just 16% of capacity in mid-2009.
In 2010 and 2011 – Australia’s wettest two-year period on record – exceptionally high rainfall helped Thomson Reservoir recover. It was kept from supplying Melbourne while other parts of the water supply network provided water instead, allowing it to ‘rest and refill’.
The following graphs show Thomson’s water inflows and storage levels since 1984.
- Inflow into Thomson Reservoir (1984-2021)
The light blue bars show how much water has flowed into Thomson each year since 1984. The green and orange lines show how the average has changed in the past 37 years: from a yearly average of 254 billion litres, to below-average inflows of 182 billion litres since 1997. It’s a similar story with Melbourne’s nine other reservoirs.
- Storage levels at Thomson Reservoir (1984-2021)
The blue line shows changes in the volume of water in Thomson between 1984 and 2021. It has been full three times: 1992, 1993 and 1996.
- Annual rainfall at Thomson Reservoir (1975-2021)
Monthly rainfall records (mm)
Last updated: January 2022
|January||160.8 (2016)||8.0 (2013)||62.3|
|February||216.8 (1994)||3.0 (1976)||51.2|
|March||170.6 (2012)||10.2 (2004)||66.8|
|April||337.8 (1990)||17.9 (1981)||73.6|
|May||179.8 (2000)||18.4 (2009)||73.0|
|June||325.0 (2007)||7.9 (1979)||105.4|
|July||198.6 (1987)||20.8 (1976)||100.7|
|August||227.6 (1990)||13.4 (1982)||104.9|
|September||236.6 (1993)||36.6 (2015)||105.1|
|October||296.5 (1976)||16.0 (2015)||97.3|
|November||200.0 (2007)||32.0 (1979)||95.9|
|December||178.6 (1986)||19.4 (2019)||80.0|
|Annual total||1467.2 (1990)||623.5 (1982)||1016.2|
Historical construction footage
Thomson Reservoir was built by more than a thousand construction workers. They were housed with their families at the purpose-built town of Rawson nearby, which remains to this day.
See photos of the dam’s construction, as well as storage levels over the decades, in our Thomson Dam album on Flickr.
Or get a glimpse of work in progress with these excerpts from our historical video collection: