Enlarging Cotter Dam

Constructed in 1912 and enlarged in 1951, the Cotter Dam provided all Canberra’s water until 1961, when Bendora Dam was constructed.

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

The Cotter Dam option looked at a range of dam sizes. The preferred size was the largest possible, a 78 gigalitre (GL) dam. The main reasons for selecting the larger dam over a smaller one not only relates to cost efficiencies in terms of yield, but also to a concern that a rock ledge at the back of the dam would make it difficult for fish to pass through at certain times. A larger dam would flood the rock ledge and accommodate the passage of the fish, including the endangered Macquarie perch.

While a larger Cotter Dam will provide additional storage capacity, if water inflows to the ACT’s catchments continue at record low levels (down by almost 65 percent on average over the last six years, and 90 percent in 2006), there will be insufficient water to fill the dam.

Hence the need to consider topping up the Cotter reservoir with purified water in parallel with the Cotter Dam enlargement. These two projects combined would further enhance the security of the ACT’s water supply into the future.