You probably haven’t noticed, but the amount of water in Lake Burley Griffin dropped late last month, and it has nothing to do with the heat.

As part of the ACT Government’s construction work on the Acton Waterfront, the National Capital Authority (NCA) announced it would have to drop the lake’s water level by 100 millimetres from 18 November.

The work involved excavating below the usual water level to construct footings for a retaining wall, and the NCA assured the lake would return to normal about two weeks later.

But now it’s about to pull the plug out again, for longer.

Um, why?

The Federal Government has greenlit a $36 million project to strengthen the dissipator of Scrivener Dam, a series of large concrete blocks designed to bear the brunt of the force of the water flowing over the floodgates and prevent it from causing erosion issues downstream.

Up to 700 new anchors will be installed in the dam’s foundation rock to a depth of around 10 to 20 metres underground, and the dissipater’s main slab will be strengthened with an extra 500 mm of concrete slab.

It’ll be the largest upgrade to the dam in its 61-year history and a necessary one.

In a study commissioned by the NCA a few years ago, engineers from the University of New South Wales (UNSW) and SMEC Australia built scale models of the dam and its current dissipater to see how it performs under “flood conditions”.

Scrivener Dam’s floodgates were closed to form Lake Burley Griffin in September 1963. Photo: Michelle Kroll.

They found “there is the potential for the dissipator to be significantly damaged during a major flood event which, in turn, could lead to the failure and subsequent collapse of Scrivener Dam”.

The early work will start in the next couple of weeks, before Christmas and before the main construction starts in March of next year. The whole project is set to be completed by mid-2027.

From March, the lake’s water level will also be dropped by 500 mm “for the duration of the project”.

For reference, Lake Burley Griffin has a maximum depth of 17.6 metres near Scrivener Dam, a minimum depth of 1.9 metres in the East Basin, and an average depth of 4 metres.

A diagram showing the proposed changes to Scrivener Dam. Photo: NCA.

What does this mean?

The NCA’s director of Lake and Dam, David Wright, says the water level will be lowered over a period of one to two weeks, slowly to “avoid causing a sudden surge of water flow downstream of the dam”.

“The NCA has consulted widely with lake users through our Lake User Group to understand the impact to activities on and around the lake,” he adds.

“The drawdown of up to 500 mm has been selected as this offers a good balance for minimising disruption to construction while minimising impact to use of the lake.”

Those around in 1963 will remember how a lake that was meant to fill by Christmas of that year ended up taking six months (due to drought) so similarly there’s no definite timeline on how long it will take the water level to rise to normal levels once construction finishes.

“This is dependent on the weather and inflows at the time,” David says.

Love Boats founder Charles Chatain at the helm of ‘Miss Pink’, one of his electric picnic boats. Photo: James Coleman.

What if you’ve got a big boat?

Charles Chastain is the founder of Love Boats, which offers a fleet of pedal boats and covered electric-powered picnic boats to the public for hire from the Kingston Foreshore.

The largest boats in his fleet, which can seat 10 people, measure 1.2 metres from the water line to the keel, but Charles isn’t worried.

“Nobody’s going to have a problem,” he says.

“I’m just going to keep telling people to not get too close to the edges – and stay 30 to 50 metres away – as you tend to see more exposed rocks around the edges. The level of the lake already fluctuates a whole lot, so there’s not going to be any changes.”

More information on the project is available on the NCA website.