Canberra's 101-year-old Cotter Dam wall is expected to disappear forever this week after having saved the capital from draconian water restrictions ''one last time'' during the 2004 drought.
It will be the second time the 30-metre wall has been submerged in just on a year following a freak weather event on March 1.
This time the inundation is deliberate and permanent. ACTEW Water said on Monday it had closed the 3.4 metre-high river diversion construction gate that was directing the Cotter flow around the site of the new 80-metre dam wall.
The original structure, one of the first infrastructure projects to be commissioned for Canberra, along with the Cotter pumping station and the powerhouse at Kingston, dates to 1912. It was increased in height in 1951.
For the past few years it has been holding water back from the construction site of the replacement dam, which is now holding water in its own right and has 20 times its capacity.
Once the 100-metre basin between the old and the new walls fills up, something ACTEW Water managing director Mark Sullivan says should happen in the next few days, the old concrete structure should be submerged permanently.
Mr Sullivan said given the old dam is only 30 metres high it was unlikely it would ever be exposed again.
If it does reappear, it will be a dark day for the ACT.
The enlarged dam is part of a $363 million investment in water security that includes a pipeline from the Murrumbidgee River to Googong and a major buy-up of water entitlements from users downstream.
It is a water storage of last resort, not a day-to-day source, and will be called upon only when there is a danger of severe water restrictions.
The need for such a back-up was highlighted during the 2004 drought when the old Cotter Dam was brought back online after more than 30 years as an emergency reserve to avert severe water restrictions in the ACT.
Big rains in 2010-11 and last year and unexpected issues with the geology of the site in 2011 pushed out the original completion date by about 18 months.
Mr Sullivan was confident all the infrastructure needed for water to be pumped from the new dam into the Canberra water supply would be in place by April.
While that capacity will be tested regularly it will not be called upon unless the Bendora Dam and the Corin Dam run low. Both are upstream from Cotter Dam and can send water to the Mount Stromlo Water Treatment Plant by gravity. Water from Cotter Dam must be pumped to Canberra.