Mystery lake floods again

The world knows about the floods that have swamped parts of Queensland, north-eastern Australia. Less newsworthy have been the heavy rains in south-eastern Australia.

Off the back of these rains, the heaviest in many years, the ‘lake of mystery’, Lake George in southern NSW near Canberra, has begun to fill.

Mystery lake? Many myths have evolved over time to explain the transient waters of Lake George. Historically, one of the most popular (and outlandish) theories is that the water comes from a secret underground spring and drains away through a crack in the earth (to China, it has been said). In fact though, there is no mystery to hydrologists: the water levels simply depend on the balance between the evaporation rate at the lake and the rainfall and water flow in its very small catchment.

When empty, the lake is so dry that its bed becomes paddocks used for grazing. When full, the waters spread over 155 sq. km. Today, the lake is about one-third full (and around 1.5 m deep), a result of drainage from the 800 mm of rain (about 31 inches) that has fallen in its catchment during the recent wet spring and summer. According to local graziers and scientists, that rainfall is not only well above the long-term average, it is the fourth highest rainfall since 1890.

One of the world’s oldest lakes, Lake George formed when the natural outlet of that valley was dammed by geological uplift, five million years ago, making it a closed body of water. The lake bed has accumulated the salts and nutrients that have flowed in from its catchment over the millennia; and when wet, the lake tends to have salty water, rich in nutrients — a potentially major influence on the area’s ecology. The waters have ranged in depth from 0 to 7.3 m since measurements began in the 1800’s.

Rich nutrients and variable water levels – Lake George is not just mysterious it is also very interesting for its fish, birds and other inhabitants.  During the late 1800s Lake George supported a commercial Murray Cod fishery which collapsed during a 50-year dry spell at the turn of the century. During the 1960s, there was an enormous population of exotic Redfin Perch in the lake and anglers were catching up to 200 in a day. But that population crashed when in the late 1960s the waters almost dried up again.

When episodic lakes like Lake George refill, algae boom and zooplankton emerge from their dormant state in the lake sediments. A succession of organisms appears and the lake again becomes a functioning aquatic ecosystem. In the past, Lake George has supported 201 species of birds, 31 mammal species, 29 species of reptiles and 12 amphibian species, and it may do so again soon.

And, interestingly, the shores of Lake George house a recently completed 140 megawatt wind farm – electricity from which powers the new Kurnell desalination plant in Sydney.

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