Lake Eyre, in the deserts of central Australia, is the largest lake on the continent. That is, on the rare occasions that it fills. Usually it is nothing more than a dry salt lake. The Lake Eyre Basin has a drainage basin that covers one-sixth of all Australia. It is one of the largest internal drainage systems in the world, and covers roughly 1.2 million square kilometres, including much of inland Queensland, large portions of South Australia and the Northern Territory, and a part of western New South Wales. To provide a sense of scale, the Lake Eyre Basin is about the size of France, Germany and Italy combined.
All the rivers in this vast, flat area lead inland. On those fairly rare occasions when there is sufficient rainfall to make the rivers flow at all, they flow towards Lake Eyre, the lowest point on the continent, at approximately 15 metres below sea level. None of the creeks and rivers in the Lake Eyre Basin are permanent: they flow only after heavy rain, which is a rare event in the arid interior of Australia. Because of the flat terrain, it takes almost a year for water to reach Lake Eyre from the headwaters. Usually, none does; it evaporates or is absorbed into the earth. Only in exceptional years is there sufficient upstream rain to provide a flow into Lake Eyre itself.
In flood years, the lake bed fills and for a short time undergoes an amazing period of rapid growth and fertility. Typically a 1.5 metre flood occurs once every few years, and a fill - or near fill - only four times each century. Long-dormant marine creatures quickly multiply, weeds and algae suddenly flourish, and fish and crustaceans emerge on a fast-breeding cycle. Huge flocks of waterbirds - numbering in the millions - arrive from around the country to feed and raise their young before the waters evaporate once more.
The last notable floods were in 1989 and 1992, each year bringing about 12 cubic kilometres of water into Lake Eyre. But these were dwarfed by the massive 1974 fill, which brought the lake's capacity to almost 40 cubic kilometres! On such a year, the nation's pelicans, which follow no particular schedule of regular movement, flock to Lake Eyre in numbers so great that their absence in coastal areas is unmissable. In fact, the pelican colony that inhabited the lake in 1989/90 was estimated to number around 100,000 birds - almost 90 percent of Australia's total pelican population. They are adventurous and opportunistic birds - trading the comforts of the coast for the harsh Outback and the chance of an easy freshwater feed. While the lake was full, the pelicans fledged an estimated 80,000 chicks - the population of Australia's pelicans almost doubled in one year!
Once the great lake dries again, the population disperses once more, flocks of thousands being seen on the northern coasts and some individuals reaching Christmas Island, Palau and New Zealand.
But after decades of research, nobody knows how the birds know when Lake Eyre has been flooded, or how they know the way there from their coastal homes, thousands of kilometres away. Adelaide Zoo research scientist Greg Johnston says there is a theory that the pelican breeding colonies act as information centres, each bird passing on information about good feeding grounds. There are other theories too, including birds having the ability to hear sonic vibrations and pick up on waves lapping at the shores and masses of water in creek systems, and magnetic compasses in their brains working on the magnetic forces on Earth.
