Melbourne Water : Water Storages : Water Supply : Water Distribution : History of the Thomson Dam

History of the Thomson Dam

On 31 July 1984, the newly-built Thomson Dam was officially included in Melbourne's total water storages for the first time. Still virtually empty when factored into Melbourne's total water system storages, Thomson's size dragged down the city's storage levels from 65% to 26% overnight.

With a capacity almost four times that of Melbourne's next biggest reservoir, the Thomson was designed as a drought reserve – to be built up in wet years and used in dry years.

Storage and inflow graphs

The commonly-held notion that Thomson would 'drought proof' Melbourne was based on assumptions about rainfall and inflows that have changed significantly.

The graphs below show how much water has flowed into Thomson since 1984 and the corresponding storage levels over this time.

They show the effect of reduced inflows since 1997, in which time the amount of water in Thomson has continued to decline as inflows have continued to be well below average.

Thomson Reservoir - Storage Levels (1984 - 2011)

What this chart shows: The blue line shows changes in the amount of water in Thomson between 1984 and 2011. It has been full three times (1992, 1993, 1996).

Thomson Reservoir - Inflow Levels (1984 - 2010)

What this chart shows: The light blue bars show how much water has flowed into Thomson per year since 1984. The dark blue lines show how the average has changed in the past 14 years - from 254 billion litres p.a. to 159 billion litres. It's a similar story with Melbourne's nine other reservoirs.