The Role of Dams
Dams have served Melbourne well since our first (Yan Yean) was built in 1857. We now have 9 in service, located in some of the wettest parts of Victoria.
But runoff into reservoirs (which is how they fill up) is now much lower because it rains less. This is made worse because the soil in the catchments rarely gets wet enough to generate decent runoff into the reservoirs.
The Thomson Dam in 1997 after being at 100% full in 1996 and at 20% in 2008.
Winter 2008 is a good example - even though we had almost 100% of the average rainfall over the catchments, we only had about 60% of the average runoff (water flowing into the storage reservoir). This was because the soil was dry from a poor autumn. Almost half the rain was absorbed into the soil rather than flowing into the rivers and streams that feed our storages.
Our reservoir storages need a steady rain over days and weeks, to wet the ground and then generate runoff.
Unfortunately, we cannot rely on this kind of rainfall like we used to. Any new dam we build in the Melbourne catchment will face the same problem.
Time is also a factor. It took well over a decade for the Thomson Dam to be designed, approved and built. This means a new dam anywhere in the State would come far too late to help with the sudden drops in inflows, like the one we saw in 2006.
Mitchell River
The Mitchell River is located in Victoria’s east, near Gippsland. It has often been cited as logical place for a dam, but there are key reasons why Melbourne Water doesn’t support this:
- Climate change - while the Mitchell has flooded recently, investing billions of dollars in another rainfall-dependent water source in the face of rapidly changing climate patterns is very risky.
- Gippsland Lakes - these lakes include Ramsar listed wetlands and to a large extent rely on the Mitchell for the health. They already suffer from blue-green algae outbreaks, so damming the Mitchell River could have serious consequences for the health of the lakes and industries that rely on them.
- Time - it could take 10-15 years before a new dam could even be built. Because of the rapid decline of rainfall and runoff into existing storages, Melbourne needs 240 billion litres by 2012.
Other considerations include the farms and towns that would be flooded as a result of damming the Mitchell. The Mitchell itself has Heritage River status and is Victoria’s last, largely untouched major river.