Why is the Blue Lake in Mount Gambier blue?
The Blue Lake in Mount Gambier turns a vivid cobalt blue each November and fades to grey-blue by about March. Warmer water causes tiny calcite crystals to form and scatter blue light.
The lake fills a volcanic crater about 70 to 80 metres deep. In spring, as the surface water warms, calcium carbonate comes out of solution and forms microscopic calcite crystals.
These crystals scatter blue wavelengths of sunlight, giving the lake its bright summer colour. In the cooler months the crystals dissolve or sink, and the colour becomes a dull grey-blue.
The city's water supply
The Blue Lake provides Mount Gambier's drinking water. It is fed by groundwater from the region's limestone aquifer, so it is carefully protected.
Visitors can walk a 3.6-kilometre path around the rim, with several lookouts. Aquifer Tours take people down a lift shaft to a glass panel close to the water.
The colour change happens quickly, sometimes within a few days, and is one of South Australia's best-known natural events.
Did You Know?
The Blue Lake is called Waawor in the Boandik language.