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After Melbourne suffered drought in the summer of 1982-1983, the population was promised that it would be the last time water restrictions would be in effect with the completion of the Thomson Reservior in 1983. (Ignoring of course the fact that we are suffering a drought now). Melbourne's biggest water catchment structure, the reservoir was larger than all the other catchments put together, effectively more than doubling Melbourne's total water storage. At over 160 metres, the dam wall is the second largest in Australia after the Dartmouth Dam, and the Thomson's water capacity is over a million megalitres. Like Melbourne's other bulk water storage facilities, the water in the Thomson Reservoir undergoes a settling filtration over a period of around three to five years. This is part of the reason that Melbourne has one of the best tap waters in the world.

The Thomson River was first considered for a dam during the drought of 1915, but the Melbourne Metropolitan Board of Works was challenged by farmers needing irrigation and other rural interests, so no plans were ever put forward. By the mid 1960's, pressure was put on the MMBW to alleviate the droughts that Melbourne was suffering every ten years. By 1973, tunnels were being drilled through the rock of the Great Dividing Range to achieve water flow from the Thomson River to the Upper Yarra Reservoir. Construction began on the dam wall itself in 1976, but it wasn't until the 1980's that earthmoving equipment and new processes of soil mechanics became advanced enough to complete the wall. After the Ash Wednesday fires, the dam was opened in 1983.


Area of catchment: 48,700 hectares
Type of dam: Earth and rockfill embankment
Maximum height of main dam from river to crest: 165 metres
Length measured along top of dam: 590 metres (saddle dam has additional length of 580 metres)
Surface area at full capacity: 2,230 hectares
Total available capacity:1,068,000 megalitres
Where the water goes: Around 18% of the Thomson River Basin water flows into the Thomson Reservoir. Approximately 46% of reservoir outflow has been used to supply Melbourne, with the 54 % has been released for maintaining environmental flows and supplying irrigation areas downstream. Water supply from the Thomson Reservoir for Melbourne goes to Upper Yarra Reservoir and then to Silvan Reservoir for distribution throughout the metropolitan area.
Date of completion: 1983
Percentage of Melbourne's total water catchment: 60.237%

Sources: www.melbournewater.com.au

The reservoir is managed by Melbourne Water, and Yarra Valley Water.

Achromatic, who works for YVW, tells me that settlement filtration is the only filtering process that the water goes through (it is of course given additives etc to ensure you're not drinking animal shit and the like though).

Chuma says: If you go up to Rawson and Erica today (towns near the dam) you will find a lot of houses have concrete driveways as the concrete trucks were dumping left over cement on the sides of the road after going to the dam and people from the towns were getting wheelbarrows and taking it back to their houses.

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