Enlarging Cotter Dam

Constructed in 1912 and enlarged to 4 gigalitres (GL) in 1951, the Cotter Dam provided all Canberra’s water until 1961, when Bendora Dam was constructed.

What is it and how can it supplement our supply?

Enlarging the Cotter Dam has long been contemplated as an option for increasing water supply to the ACT and surrounding region. In 2004-05, after considering over 30 new water supply options for the ACT and surrounding region, ACTEW undertook Future Water Options, a study to further examine three shortlisted options, one of which was the enlargement of the Cotter Dam.

The Cotter Dam option included consideration of a range of dam sizes. The preferred size was the largest possible, a 78 gigalitre (GL) dam. One reason for selecting the larger dam over a smaller one relates to the ability to get more water for less cost. Also, there is a concern that a rock ledge at the back of the dam would make it difficult for fish to pass through at certain times. The advantage of the larger dam is that this would flood the rock ledge and accommodate the passage for the fish, including the endangered Macquarie perch.

ACTEW is working to remediate the Lower Cotter catchment to achieve significant water quality improvements that are required to manage the effects of the 2003 bushfires.

Responding to climate change and variability

All of the options outlined in the Future Water Options studies were based on some key assumptions. ACTEW identified that there had been a fundamental change in one of these assumptions; climate variability and climate change. Based upon work by CSIRO, it was predicted that by 2030, inflows into storages would decrease by 30%, on average. Between 2001 and 2006, however, inflows decreased by more than 60% and by nearly 90% in 2006 alone. This prompted ACTEW to review previous studies and look at further options to secure supply, including the Enlarged Cotter Dam option.

How much will it cost?

The cost estimate is approximately $119 million for the dam and associated works, plus allowances of $4 million for clearing and site preparation, $2 million for pipelines, $15 million for the pump station and $5 million for miscellaneous works, giving a total cost of approximately $145 million.

How long will it take?

The Dam is expected to take 3 years to build.