Lake Eyre Basin

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Lake Hart, one of the smaller lakes in the basin^[1]

The Lake Eyre Basin is a drainage basin that covers one-sixth of all Australia. It is one of the largest internal drainage systems on Earth, and covers 1,140,000 square kilometres, including much of inland Queensland, large portions of South Australia and the Northern Territory, and a part of western New South Wales.

All the riverbeds in this vast, mostly flat, arid and semi-arid area lead inland (not towards the sea) and on those fairly rare occasions when there is sufficient rainfall to make the rivers flow at all, they flow towards Lake Eyre in central South Australia.

Lake Eyre itself lies 10 metres below sea level, and usually contains only salt. In flood years it fills and for a short time undergoes a period of rapid growth and fertility: long-dormant marine creatures multiply and large flocks of waterfowl arrive to feed and raise their young before the waters evaporate once more.

None of the creeks and rivers in the Lake Eyre Basin are permanent: they flow only after heavy rain–a rare to very rare event in the arid interior of Australia. Average annual rainfall in the area surrounding Lake Eyre is 125 millimetres (5 inches), and the pan evaporation rate 3.5 metres (about 11 feet). Annualised average figures are misleading: since 1885 average rainfall over the 1,100,000 square kilometres of the Lake Eyre Basin has ranged from about 45 millimetres (less than 2 inches) in 1928 to over 760 millimetres (30 inches) in 1974. Most of the water reaching Lake Eyre comes from the river systems of semi-arid inland Queensland, roughly 1000 kilometres to the north.

Because the Lake Eyre Basin is almost flat, rivers flow slowly and frequently split up into floodplains or multiple braided channels. Water is lost to evaporation, to seepage, and in the many ephemeral wetland systems, with the result that downstream flows are typically smaller than upstream flows. Only in exceptional years is there sufficient upstream rain to provide a flow into Lake Eyre itself.

To provide a sense of scale, the Lake Eyre Basin is about the size of France, Germany and Italy combined. It is roughly the same size as the Murray-Darling Basin (which drains inland eastern Australia and is responsible for a large proportion of the continent's agricultural production) but has vastly less water. Nevertheless, the entire flow of the Murray-Darling would be insufficient to fill Lake Eyre, merely keeping pace with evaporation. In contrast, the flow of the Mississippi could fill Lake Eyre in 22 days, that of the Amazon in just 3 days.

The basin began to form in the early Tertiary (about 60 million years ago) when south-eastern South Australia started to sink and rivers began to deposit sediment into the large, shallow basin. The basin is still gradually sinking, and still gradually accumulating sediment. For many millions of years, the Lake Eyre Basin was well supplied with water and largely forested. About 20 million years ago, large shallow lakes formed, covering much of the area for about 10 million years. From that time on, as Australia drifted further north and the climate became gradually more arid, the lakes and floodplains started to dry. Only in the last 2.6 million years did the onset of the ice ages bring about the present climatic regime and the consequent fairly rapid desertification of the area.

Several of the major Lake Eyre Basin river systems are well-known.

The Finke River, starting roughly west of Alice Springs is thought to be the oldest riverbed in the world and although it flows for only a few days a year (in many years it does not flow at all) is home to 7 species of fish, 2 of which are found nowhere else. The waters of the Finke disappear into the sands of the Simpson Desert and are not definitely known to ever make it as far south as Lake Eyre, although the story is told that this happened once early in the 20th century.

The Georgina River system originates on the Barkly Tableland, near the Northern Territory-Queensland border, north-west of Mount Isa and not far south of the Gulf of Carpentaria. In this relatively humid northern area, rainfall can be as high as 500 mm (20 inches) per year and evaporation as low as 2.4 metres (about 8 feet). The Georgina flows through innumerable channels leading south through far-western Queensland for over 1000 kilometres, eventually reaching Goyder Lagoon in the north-eastern corner of South Australia.

Australia's early bush poets immortalised the Diamantina River, making it a symbol of the remote outback. It too rises in northern Queensland, roughly between Mount Isa and Winton, flowing 800 kilometres south and west through Birdsville and the Channel Country to join the Georgina at Goyder Lagoon (and then, if there is sufficient flow, down Warburton Creek towards Lake Eyre).

Of all the Lake Eyre Basin river systems, however, Cooper's Creek is by far the most famous, in particular because it was along Cooper's Creek that the explorers Burke and Wills met their deaths. It rises in the form of two central Queensland rivers, the Thomson between Longreach and Charters Towers, and the Barcoo in the area around Barcaldine, about 500 kilometres inland from Rockhampton. Cooper's Creek spreads out into a vast area of meandering ephemeral channels, making its way roughly south into the far south-west corner of Queensland before turning due west into South Australia towards Lake Eyre. It takes almost a year for water to reach Lake Eyre from the headwaters. In most years, none does: it is absorbed into the earth, goes to fill channels and the many permanent waterholes, or simply evaporates.